I could never just use "any old amp" as my main thing live, and I prefer not to do that for practice either but then I'm a little more lax about it. I know there's a myth here that "the audience doesn't care"/can't tell good sound from bad, but even if that were true which it's not, *I* care what I sound like, at the very least because when I'm happy with my sound, I play better and enjoy playing more.
On top of that, I'd ALWAYS rather have a little too much rig on stage than not quite enough. Certainly until you get down to VERY low volume/small gigs, you never know when you might need just a little more to be comfortable and/or heard. So at least for the kind of stuff I play, if there are live drums and electric guitars, that basically means 4x10 or equivalent and maybe 300+ watts.
If you're going through a nice pre or DI into a good PA with subs that's properly dialed in (hopefully by someone outside the band who knows what they are doing)(ie is getting paid), you might not miss anything in terms of sound quality from the audience point of view. On stage could be a different matter. With guitar you can get away with a lot, but bass needs a certain "saturation" level to sound right. I dont mean overdrive, but a certain amount of low end for the mix to be where it should be, and you are less likely to get that with a "small as possible" stage rig. So then you may find yourself cranking mids which could take you WAY off target in terms of what the audience is hearing. Better to just bring the muscle you need in the first place. Obviously you also need to not be blasting loud, but that goes for any instrument no matter how much gear you bring. And I never see people talking about drummers using lighter sticks, thinner cymbals, etc... You have guys playing pop covers with the Lars Ulrich circa 1989 style kit (albeit probably just a 4 or 5 piece) with giant shells and 10 lb 26" rides and whatnot and smacking the hell out of them with 2Bs.
So as a sound guy, I'm happiest (for a typical club gig in a 100-300 capacity or so room), when if the guitarist is using a 25-50w 1x12, the bassist has the equivalent of a 4x10, with about a 300-500w head, and the drummer is using smaller, thinner cymbals and thin or medium sticks, with reasonably sized and tuned shells. If that's the gear complement and the band is self mixed well on stage, I can use the PA to just do a little "rounding" in the mains and make everything bigger. And if that's the case, I can make your band sound about as close to CD quality as is humanly possible in a bar/club setting. Honestly, even having electronic drums and guitars and bass all being direct presents a lot of other challenges because then you're fighting a certain artificial sound, even though you can basically make the volume nothing. Because a band playing live drums and instruments through amps is always going to sound like a live band by definition. Just like you can't make a bad sound on a real B3 into a real Leslie, because by definition it is what it is. Whereas with digital emulation, it's easy to go into the weeds into things that don't sound that great or just aren't really authentic.
