in a 3-4, it's essentially 3 guys who could play 4-3 DT, and 2 guys who can play 4-3 DE. So it really is almost a 5-2.
I think part of the reason for the recent dominance of the 3-4 is because not a lot of teams run it, and guys like NTs and DEs were slipping further in the draft than they would have if there was an even amount of 3-4 teams. With more 3-4 teams, this trend will probably stop, making it much more difficult to have a good 3-4 defense, since there are only so many nose tackles to go around, and that's the most important position by far. We're probablygonna see more Atlanta's and Houstons(when they ran 3-4) than another New England, Pitt, or SD anytime soon.