I will tell you why the big amps. It is because of the sound and the air that the speakers move. No..it doesn't have to be 120dB, but a 4x12 cabinet at 105dB sounds to ME much better than a little combo. And I say 105dB is just fine.
I used to play in a band that played a lot of 500 seat clubs, high school proms, stuff like that. We NEVER mic'd anything and had a heavy hitting drummer. A half stack, much less a full stack was just fine in those places. It wasn't about ego, etc. But try to fill up a gymnasium with a Roland 60 cube and you see what I mean.
At smaller clubs, we just always turned our amps around or just took half stacks.
Playing Led Zep covers with a Peavey Classic 30 just doesn't cut it.
I also believe that a 50 watt amp is about min with a LOUD drummer.
NOW>>
if you have a GREAT engineer, a GREAT monitor mix, I will CONSIDER a small amp.
At my church, I run three amps...all about 30 watts each, except for a 130 Peavey that handles the guitar synth. I turn the amps off to the side, and where I stand, the mix and sound and volume level is PERFECT for me. Unfortunately, church soundmen usually aren't the best, and I know the FOH suffers for it. However, I only play for God in that instance, and leave the FOH sound ENTIRELY up to the soundcrew to do what they think is best for the congregation. I definately do what is best for the praise team as a whole and am eager to work with my sound for whatever is needed.