I played bass through someone elses Line6 Lowdown bass amp at a gig the other week. I'm sure a roaring Ampeg stack would have sounded better, but the Line6 sounded at least very good indeedand was super easy to get set up and into the mix (line out to the PA, onboard speakers for sound onstage, and that speeds up the soundcheck pretty handily).
So I was favourably impressed by the value-for-convenience option. Even if I had the money for a roaring Ampeg stack, I'm not sure how the hell I'd transport it; I'm pretty sure I wouldn't have wanted to wrestle it up the flight of stairs at that venue! ;)
All things have their place, I suppose. Tool for the job. Were I a rich rock star with piles of roadies and techies to worry about this stuff, of course, it wouldn't matter 8)