Level6 was to prolong the length of the game. I've seen a couple of people on these forums, but not a soul in game, who has actually accomplished all 6 => lv6.
Agree with Crusader, no need for commandos to be able to bring down a FP with 1 bullpup clip, thanks. You could balance it by bringing in another skill level, but quite frankly suicidal is tough enough, I wouldn't want to see the game become the clash of the titans.
If people have farmed XP on levelling maps to get to level 6, and are now thinking "so what now...Ive 'finished' the game"... then I have no sympathy for you. I've been playing the game 4-6 nights a week since April and I have 1 x lv6, 2x lv5, the rest lv 4. So after the better part of a year I still have lots of playing left to accomplish 6xlv6 through natural perk progression.