We are, to be practical, going to have to pay attention to minimizing bandwidth. My point is we want to do that by keeping out robots, not strangers. Strangers who want to use our art are friends, not enemies. Because, even if strangers outnumber forum regulars five to one, they still won't use a tenth the bandwidth that unrestrained robots and hotlinkers will.
Doesn't a login system already inhbit hotlinking quite effectively?
Yes, but do you really want to require someone to log in to *browse* the site? (Remember, hotlinking will be to the thumb-sized or palm-sized images on the index pages. Nobody hotlinks a fifty-megabyte hi-res texture as a forum avatar.)
If you put the index pages and galleries behind a login wall, the site is effectively opaque to visitors. Nobody registers for a web site that presents nothing but a "please register" front page.