My reasons for playing pump, not in order:
1. Don't need as much paint=lower cost
2. A decent pump typically will cost less than a decent semi/electro=lower cost
3. Don't need batteries
4. Less maintenance issues, i dont need to spend much time fixing it...of course i shoot a phantom so my down time so far has been zero
5. Makes me play more methodically, i dont have the fire power to get a hit by volume so aiming/stealth is needed.
6. It's generally a lighter set up...a smaller tank and hopper can be used without hurting your play style or time in the game.
7. Some game's/tourneys require a pump. Outlaws and Lawdogs wouldn't be the same with semi's/electro's
All that being said I still think its fun to use a semi/electro on occasion. Optimally id like to have a gun in every catagory and eventually ill get there.