If you can't wear trainers for a leisure activity, what are they for? I don't think they'll get any more ruined than going out in the rain or snow. They're not made from some rare, delicate, priceless material are they?
As far as heels go, some people will wear heels, some sandals, some flip-flops (although they may not be the best choice). They all run the risk of getting stepped on.
Personally, I would wear flat shoes (they've got to be better for dancing and comfort.
This year, I will be wearing those strap-on, jesus-type sandals (velcro, not buckles) that are all over. I would wear trainers as I did last year, but my feet sweat profusely in the UK, let alone the heat of Ibiza.
I think, as a rule, you should wear whatever you think is the most practical and what you feel most comfortable in (physically and mentally).
Personally, I have never looked at a girl and thought, hmmm... nice shoes! You can't see them when it's real busy anyway.