you play call of duty 4, so look at it this way -- all the added points you get for challenges towards ranking up add incentive for you to switch up your gun. yeah, you might be nice with the M16, but once you got your 150 kills with it and got all your challenge points, you might wanna do the same with the G36C. you'll keep playing because you'll check your challenges and see that you need 8 more kills with the MP5 to get 500 points towards a new rank. that's what the achievement/gamerscore system is all about.
also, in a game like rainbow six where you get achievements for hitting certain ranks online, you get to see who cheated and who's legit. if someone got the Elite badge, but not the Best of the Best achievement, you know they glitched it.
i read in GameInformer magazine earlier this year with the editor pleading with Sony to make moves to get something like that done on their console. it doesn't cost the developer's anything extra to accommodate achievement statistics.
replay value ftw