Midcaps and locaps.
Most people play airsoft for the exciting experience of simulated infantry combat. Without the necessity of reloading to break up the pace, running and gunning for hundreds or thousands of rounds straight gets kinda dull.
That, and every time you reload, you are drawn a little deeper into the whole tactical feel of a firefight. You more feel like you're actually in combat, since by having to reload, you are doing all the little things people do in real combat.
"The Bird of Hermes is My Name, Eating My Wings to Make Me Tame."