eh...like most responses on here, i have to say that it depends on the application
for instance, if i'm recording some singer/songwriter sort of stuff that has just vocals and acoustic guitar, you can bet your ass i'm going to record the guitars in stereo, in order to get that lush, full, character
if i'm trying to squeeze an acoustic passage into a mix that already has 20-30+ tracks to its credit, then chances are i'm going to get a mono signal
i guess it really depends on how critical that single instrument is to everything else that's going on...