I think it depends on the size of the PA. A simple, small scale vocal only PA is traditionally owned by vocalist(s). However, when you add things like monitors for the rest of the band and increase the size of the PA, it can get tricky (and expensive). I'm fortunate to own a high quality PA (have been playing a long time and have built up to it). So when I take it out for the bands I play in, I expect a fee - not necessarily a large one, but it's reasonable that, if they benefit from my kit, they should pay me. Having been in bands where PAs are jointly owned, I suggest that the component parts should be owned individually by band members. It's much easier, should someone leave, for their replacement just to provide whatever they took with them -- mixer, power amp or whatever - and saves all those arguments about buying people out, or worse, selling stuff at a loss and splitting the money, only to start again.