I've never used anything under 12s - nor would I.
It depends upon what guitar and style you play, but I do a lot of fingerstyle, some of it quite quick, and I find that lighter strings, apart from sounding "lighter," bounce around too much when doing the fast stuff.
Bearing in mind that with an acoustic guitar, the amount of sound you generate is in direct proportion to how hard you hit the strings, unlike electrics, loose strings just don't work for me.
I also tune down a bit to alternative tunings and do that with 11s or 10s on and you'll probably have some buzzing happening.
Cheers