For guitar, that's all true. 100 watts will be overkill for most applications. Of course, that amph might sound awesome, so that's a reason to have one even if the power is unnecessary. Joe B. doesn't use all his amphs at the same time, much less at full blast. He also has plexiglass in front of his cab to keep the stage volume manageable.

Bass, on the other hand, requires a ton of wattage. 100 watts won't be enough even for a smallish venue, unless you're going into the pa system. I used to use a 50 watt combo, but I had the only electric instrument and the drummer showed restraint. We never played any place huge either.

When I tried to get a metal band together, you couldn't even tell I was there.