I'm with -MAD-SARGE.
Let's just assume for the sake of conversation that I'm running an AEG that uses magazines, like an M4.
In a perfect world, that AEG would be as accurate as the shooter, and it would only take a few well-aimed shots to hit the target. But, it's not perfect and variances such as imperfect BBs, improperly adjusted HopUp, and even dirt in the barrel will cause different trajectories. Add to that the fact that airsoft BBs are affected more by the wind, foliage, and other variables than bullets and you have the answer to your question.
Basically, it takes MORE BBs to hit the target.
I like TM Standards for the M4 (68 rds), not only for the reliability, but for the BB count.
Now, I typically run my CA249, with over 1K rounds in the drum, and I use it like a support weapon. With airsoft machine guns, I find a 5 : 1 ratio (BBs fired : dudes hit) to be perfectly acceptable, hence my magazine capacity.
All "realism" aspects aside, there are severe drawbacks to High Cap mags. For one, when you're out I can hear the spring unwind. Typically, guys that run winding hicaps have the muscle memory to "wind" rather than reload when they start involuntarily dry-firing. Those are extra seconds for my bros to give you the chicken pox.
It's a personal preference, but I think you'll find that Hi Caps are mostly used by newer guys that don't have a 2nd Line Harness yet.
For most games, I don't know of any rules against it, but I've been away for awhile.