As for me, if a system's response doesn't reach 60Hz flat, it sounds lacking. If it can reach 40Hz flat, then it sounds pretty good and full. And the futher down it reaches, the better, because I can really notice the difference between a speaker with f3 of 30Hz and one with f3 of 40Hz. The 40Hz speaker sounds full, but the 30Hz speaker sounds even more so.

That's just a matter of quantity though. As long as we're hitting 40Hz, quality becomes more important to me. Adding subs adds bass sound sources, and this smoothes room modes. So the benefit of adding subs isn't just about extension, it's also about sound quality.