There are advantages to either defense and frankly you can run an aggressive 4-3 which is just as effective as a 3-4. You will note that while some teams are going to a 3-4, teams like New England are going to more of a 4-3.
Why are they doing that??? It really goes back to the age old argument of whether you should fit your scheme to your personnel or find the personnel to the scheme you choose to run.
If you can find a 330 lb nose tackle who has the power and speed of Ngata and you can find two DEs who are in the 300 lb range who are athletic and quick, you are set for the traditional 2 gap 3-4. Can you get by with guys who are not ideal? Absolutely, there are guys in the 300 lb range who can handle the nose tackle role and guys in the 280 range who could handle the DE role in a 3-4.
Why doesn't everyone go to a 3-4?? It is harder to find draft picks who can fit the 3-4 than it is to find guys who fit the 4-3. Why?? Most college teams run the 4-3 and the tendency of late has been to find smaller quicker guys who can rush the passer and keep up with all the no huddle spread offenses which are cropping up everywhere. Plus most college D linemen are more used to a one gap attacking style of defense rather than a 2 gap read and react style. How many quintessential nose tackles can you find who could run a 3-4 in a particular draft?? 1 or 2 at most?? And how many of these guys actually show up in free agency given the best ones get snapped up in the first round and teams lucky enough to find a good one keep them.
Why do the Panthers run a 4-3 rather than a 3-4 right now?? We really don't have the personnel to run a 3-4 right now. Are we one or two guys away?? Actually not. And I would disagree with the OP that our secondary is adequate ( and it isn't the corners).
Who is the most important guy in Baltimore's and Pittsburgh's secondary that when they are out instantly changes the defense significantly. That would be the safeties- Ed Reed and Troy Polumalu. Reed is a free safety while Polumalu is a strong safety Both of those guys are arguably one of the most important players in their back seven. While they serve somewhat different roles, teams that play the 3-4 and play mostly a fire zone concept have to have great safety play. What position is aguably average at best on our team right now?? Safety play at both positions.
Do we have the linebackers to run a 3-4?? We do have some that would transition such as Beason and Gaithers and maybe Anderson at the ILB positions. But honestly do we have great candidates at the OLB position?? Most players we have been discussing are actually DEs like Johnson and undersized DEs or tweeners like Norwood. Could they do the job in time with opportunity to learn the position? Or could Johnson be an OLB like Harrison for the Steelers who rushes the passer most of the time. Absolutely. But we are likely 2 OLBs away because you need both starters and suitable backups in the event of injury.
Do we need a NT?? Hard to say until Edwards come back next year. But if we could find a NT in the first or second round next year who would be a dominant players for years, it would be a great pickup. Whether we stay 4-3 or go to a 3-4 we need a dominant NT. If Edwards is that guy, so much the better.
How about the corners. Actually I think the corners would be okay if the safety play were much better. Particularly if Hogan pans out as hoped. Plus pressure on the opposing QB makes decent corners better.
So do we have the personnel to run a 3-4. Actually we do. Do we have the personnel to run a great 3-4 right now??? No we don't. We need upgrades at several positions or guys to improve and expand their skills sets so they can do it more or do it better.
And before people start posting that we have the guys for the 3-4 by looking at weight and height of teams like Baltimore or Pittsburgh and saying we have guys with the same size and weights, get real. I can find a bunch of guys who are 6'3" and 330 lbs in the stands, but it doesn't mean they can start at NT for us. In other words every 4-3 linebacker who is 245 lbs can't play ILB or OLB in a 3-4 just because a well known 3-4 team has a guy who is in that range playing OLB.
Just my 2 cents...
Common sense does exist around here.