I gotta back the Reverend on this one. You'll find a pistol more useful in CQB: it's smaller and easier to drive in tight spaces. You can also use it for Pistol/Shotgun Play.
It's a second gun in case you run out of ammo mid-game. In most cases, the fastest reload is another gun. You can also switch to it if you want to change your play style mid-game. Try lugging around 2 M4 weapons, no matter how small, and let me know how that works out for you.
Third, most games have minimum engagement distances ("MED") for different guns. For long arms, it's often 20' and up. Pistols, on the other hand, may have no MED or something like 2'.
Last, but certainly not least, GBB pistols are just a heckuva lotta fun to shoot. The recoil and noise add to the experience.
