In reply to itsmystronghand:
Missed this thread while I was away but ...
IMO most attempts at rationalising motives are missing the main reason, which is that, in the early days especially (but still very much so today,) climbing walls were extensions of the crag environment. People usually would, and often still do, get changed at the foot of the wall instead of trotting off to get changed in private. People would keep their bags with them, often with drinks and snacks on hand for when they wanted them. In short, they behaved much more like they were at a crag than like they were in an indoor sports hall.
Gradually this attitude and behaviour has eroded, particularly in places where many of the indoor wall users rarely if ever get out to crags, and workout gym or aerobics class norms have started to take over. However, while it's definitely a continuum and the direction of travel seems to be clear, such walls are still in the minority in most of the UK. Also, the most high profile walls for wads very much retain a crag or bouldering area feel, rather than what many climbers would see as the more prudish and behaviour-conscious feel of gyms or indoor sports halls.
Taking your top off, therefore, is something most people would do as they would at the crag, simply because it's more comfortable that way when it gets a bit warm. The idea that most people with tops off are deliberately showing off is quite laughable in that context, although I don't doubt there are some individuals who take tops off more readily for that reason.
I find the often-expressed aversion to tops-off in walls to be an indication of how far climbing has reached into mainstream sport and mainstream society, bring with it norms from non-climbing activities and environments. I understand why people would feel that way, and why they would explain it as strutting and show-off behaviour, since that would be a likely explanation in a gym, a yoga class, or on a badminton court. Swimming has its own tradition and associated norms, developed no doubt originally from swimming norms outdoors, and we've become accustomed so much to its norms of near-nakedness that we think it's odd when people don't conform to these very different conventions.
I think it's a little sad that climbing is increasingly seen, among climbers as well as non-climbers, as having more in common with indoor sports than with outdoor activities, but given the huge growth in participant rates and the great opportunities for indoor climbing fun we have now, I accept that some of this is inevitable. But that's not to say I think it's right - I'm convinced the opposition to tops-off in walls is pure prudish convention, but then again when I started to use climbing walls they were really just indoor crags, so perhaps I would see things that way.