As for why I play pump...it's purely a way to mix it up. I'll play nothing but stock for a while, get bored, and use my eNMEy for a few months. A little while later, I'll decide I want to use my Sniper for a while. When that gets old, I'll break out a pistol. I'm currently on a hardcore mech 'Cocker kick; it's only a matter of time 'til that gets boring and I go back to one of my pumps.
Mixing up play styles keeps the game fresh and helps me not get burnt out.