Impossible to reach a conclusion or maintain a stance in this one as none of us are chemists who have or could ever sonically test the two finishes in a straight up, objective comparison.
I will say this, and it's just personal opinion. If I were a major guitar manufacturer, or a little guy for that matter, I doubt I would say....."Hey, we'd like to use nitrocellulose lacquer, but it takes too long, costs too much money and puts us at odds with the EPA, so we give you a UV polyeurathane finish. It just easier!" So they say it's harder (no doubt) and equal to or better than nitro. I still haven't figured out exactly why one would need a UV finish on a guitar? I've never had one lay out in the sun, and none of mine are parked next to a window. :?
I do buy Carl's logic that wood doesn't need to breathe, but feel that it is entirely possible that nitro might allow wood to expand and contract better than plastic, and if that's "breathing", or just expanding and contracting....well . We have a sizable contingent here who honestly believe that a played guitar sounds better via that logic, and I just don't know, so I certainly can't disagree. And if vibration and expansion and contraction are what make the top go round (and sound better over time), I'm all for it.
I suppose I still find myself in the pro-lacquer camp.
West