Think I'm going to pee against the wind, here. But I do prefer my models clothed. As someone said before, many girls (and boys, but that's not the point) look better when not fully nude. And even for those who don't, I tend to find it disturbing when the 'lee is naked, mixed signals if you like. I don't mind that much when the 'ler is naked, but that's probably because nobody really looks the 'ler 🙂
That being said, there's a few models where topless is ok (first comes to mind the lovely Rachelle from UKTickling), but I wouldn't say nudity gets things better for me.
Another reason why, I suppose, is some studios just get lazy when they use naked models. TickleAbuse comes to mind, especially.