~And it sorta goes along with what I theorized about in 1 thread (RE: some vast attempt at having dual interchangeable ILBs in NE).    
And some are gonna blow me up for sayin' this...but by deploying dual ILBs- ahh, Mayo ain't in that picture...

Here's WHY, the risk/reward and future:

Mayo's contract's up next year...  And he will get paid, paid, and lemme RE-emphasize: Paid BIG.  But sorry, the key to the success of a having 2 3-4 ILBs, 1 WILB (Mayo) and 1 SILB (Spikes), Isn't the 1 specific guy...and just how nasty he is, or might be.  Nope...It's both.  Think the year before last (2009-2010), and how good Mayo looked THAT season...he didn't.  And it wasn't Mayo not living up to the challenge...It was that Gary Guyton was NE's SILB (early on, this was esp. noteable).  You NEED=100% NEED, a banging SILB...and ya always need for the 3-4 decent sized D-Lineman, But in terms of Mayo getting an open-look on a ball-carrier, A player who can, and willingly wants to play that SILB spot, IS a Must.

The contingent is having some nut-case SILB, who takes on the added O-Line(man) blocks coming further out and into the LB corp, As there's 1 less D-Lineman (3-4 vs 4-3). 

YET, What IF...What IF: Pats decided to deploy, not 1, but 2 BIG SILB types...  There MIGHT be some who would make a concern RE: perfect-form tackling, of that weak-side ILB (i.e. Mayo)...but really, tackling ain't the issue, c'mon.
THIS would be the greater one:  That, There may be some concerns RE: Speed on dropping back those now, heavier/bigger SILB types on passing downs...  And Spikes's 40 WAS an initial concern among some (yet, after watching his read/react instincts, play-diagnosis skills, and ON-field speed...That's LONG gone).  But what if ya had 2 SILB, ya know?  Might this speed drop-off be further enhanced and amplified (as oppossed to a smaller speedier WILB instead).  A:  Well, just go ahead to The NFL's 2012 ILB prospects on Walterfootball.com, and see for yourselves...

~Top 8 guys!  Something like 8! of the ILBs comin' out next year: ALL-over 6'2-upwards of 6'4.  BIG boys, too!  When Mayo came out, he was like 220-230lbs in some listings...  These guys are just about @ 240-255...and that matters.  Check their 40's out and TFLs and Sacks, o.k.?  Now, Mayo ain't providing an inside pass-rush threat no matter what...sorry; He'll get caught up in the trash of the lines.  BUT, THESE guys (like Spikes) DO have the size, AND on field quickness to see that crease and be strong enough and quick enough, to get through and exploit that opening.  AND by getting just 1 of ANY of these big & good ILBs, you don't NEED to take a gamble on hopefully, begging to move up into the Top 10 Draft picks, in order to secure some superstar All-world Collegiate 4-3 DE w/ 20TFLs and 15 Sacks, and then running the even FURTHER risk of transitioning and changing his entire position from 4-3 DE to 3-4 OLB...  Nah...not worth it.  Not when ya can get some Big SILB type, with LESS of that pure-pass rushing skills, BUT (here's the pt): JUST enough of a backfield thread w/ 10-12 TFLs and 5 or so Sacks, For HIM to be accounted for...

NOW...ya got something.  Because NOW, ya got not merely 1 WOLB, no matter how good he is (let alone IF you can ever find him)- 1 WOLB whom ya KNOW which side he's coming from nearly 100% of the time...  INSTEAD, ya got 2 SILB and 1 WOLB, ALL of whom pose at least SOME semblance of a lesser, but VERY real threat...  A threat which no can be exploited when you're shifting your D-Lineman over to that weakside, and shifting your 4 LBs over the other way...  Which ILB's coming?  Both...who...Are they gonna further try to collapse the pocket from the middle (b/c they have the size)...find an inner crease in a 4-3 under, or over?  Is either interchangeable ILB now gonna overwhelm the O-Line better on a fire/zone-blitz (yes)?  Are the ILBs gonna cross over?  
  THAT trade off...is pretty freaking good in terms of More versatility (what you can do with whom and HOW/different and number of ways) , LESS single (1 specific player) Demands & etc (i.e. affording one), FAR Less concerns & attempts to find the next TRUE freak 3-4 DE (Seymour)/Less concerns in getting a freak Collegiate 3-4 DE-3-4 OLB transfer prayer McGinest)... 

And FINALLY, 2 serious questions and shortcomings of your 3-4 Defense...?  Remember?  1 /> Exploitable to the interior power rushing game (MMM, not so much so w/ now 2 guys the size of the typical 1 SILB type).  2 /> Pocket-Collapsing OF that center D (i.e. Even a great NT no longer gurantees even remotely the same inside push to collapse a pocket from the center, as 2 4-3 DTs do... And Why?  B/c teams like The Colts and Packers, and more & more-Have now learned to have whichever OG that true 3-4 NT angles towards <besides that OC he's dead-on /> And to have whichever OG, simply pull, molly-block I believe the term is, And just KEEP that NTs direction going in the angle he was originally flowing towards in his OWN angle choice).  And so now there's a big V-crease seam for that QB to pass outta 1 OT dealing w/ 1 DE and on the same side that NT you're further helpin' him pull towards, ya got your OC and OG pushing him further along in the same direction he originally started angling towards.  On the other side?  Ya got an OT and OG (who has a better interior angle on this lone remaining 3-4 DE).  So they can better pull wide the opposite way, ya know?  SO: Perfect crease...  Buuut, by doing this, it leaves exploitable openings to the interior passing rush threat by your ILBs.  If ya got just 1 WILB and 1 SILB...ehh, Chances are ya can still get a mitt on that weakside guy IF he came up the middle.  Strongside guy=Just 1 guy, so better accounting for him w/ O-Line awareness and movements as play develops...and each guy has their own specific jobs/requirements.  BUT 2 interchangeable ILBs the size of SILBs???  Hmmm...

Anyway...  WHEN BB didn't get another SILB type, and w/ Mayo currently STILL a mainstay (and a good one)...  I'm thinking, HowInTH is BB gonna provide any pass-rush, let alone an interior pocket collapsing one, or a decent enough weakside one, let alone have the personell balanced enough to do something like a zone-blitz on 1 side (wherein that OTHER side, NE's personell might just get killed IF and when the QB read which side this zone-blitz was coming from, and headed the direction of his play-call, the other way)???  

I'm thinkin', HowInTH is BB gonna offer ANY-thing like this?

Well, Here's HOW:

WR         TE   OT   OG   OC   OG   OT               WR
DMC                TWa       VW         AHy             LBodd  
           Ninc                                     Cunning
                            JM          BS
...by shifting to THIS form of a 3-4 "Under" Defense...

Spikes and D-Line's shift counterbalancing the weak-side.  The shifting of your 4 LBs (w/ your SOLB and best tackling/physical CB) counter-balancing the strong-side.  Haynesworth's as your best D-Line pass-rush threat (WITH the size...i.e. not Mike Wright)-more 1-1 looks.  VW right at 0 dead-eye on the OC (or 1 speck over as well).  Warren, as your self-less best run-stuffer, "no hastles" (cough), set-up on giving that initial stance and need for taking on 2 O-Lineman (by shifting).  LBs moving to strongside, offers zone-blitz concerns given that The D's just set-up heavy to go that way (more LBs, and w/ VW dead on OC, he CAN decide to angle THIS way...you just don't know).  YET, sheez, Your set best pass-rush specialist 3-4 OLB AND your better 3-4 DE pass-threat (Haynesworth & Cunningham) have a more open look w/ less offensive players to impede THEM... (yet the way it's counterbalanced, ya can't run or whatever, towards the Strong-side...NE's LB corp is already edged over that way).