There are many practical reasons for putting a dedicated mic on any part of the kit. For the hihats takes priority because:
> it is the most often hit among the cymbals. More events/hits can be counted on the hihat, than the kick, snare, toms, crash and rides.
> it's has the least volume when struck. More often than not, the stick is only dropped or bounced on the hat. the stroke is gentle, but with a closed hat,volume is low... compared to the crash and ride, the strike is on these two is harder... you can watch Discovery channel on how much the cymbal practically warps and bends...
> tama ung nag comment re budget. Many opt for the entry level mic kits of 4 pcs. (eh yun lang pera nila eh, kaya wag natin sila la-itin). If you can afford to have the 8-mic drum kit, then congratulations! It may create bleed problems, but there are solutions for that too...
> i have heard engineers who simply amplified the drums using a kick mic, a snare mic, and 2 overheads condensers. I thought he did great job!