Anything below 30 is shit and inconsistent framerates also suck, so if you have to lower the res, remove some texture detail/particle effects or whatever to make a game run at a consistent 30 or preferably 60, do it. As far as limited hardware goes, I'm somewhat of the mind that HW limitations inspire creative solutions when talented dev teams are involved, but I also agree that more powerful HW allows devs to spend more time on gameplay, story and general content rather than optimizing a game to run on weak hardware. Companies don't have as much pressure on them to release COMPLETE games these days because they can just patch an initial buggy release which is a shitty trend. If Gran Turismo 4 can run at 480p(upscaled to 1080i too) on a fucking PS2 @ 60fps(I think), today's games should at least be able to push a consistent 720p/60fps or at least 1080p with a rock solid 30fps. In short, more time needs to be spent optimizing games and less time needs to be spent on taking shit out of games and selling it as "DLC".(had to shoehorn that in)
Like I said earlier, 60fps is very much needed in games like shooters, racing games and fighters that require quick reactions, but a consistent 30 is generally more than playable. Modern LCD & Plasma displays generally have much more input lag than old CRTs, so 60fps is also needed to minimize that kind of thing.