The reasons I use Arc (well, not these days, due to incredible lack of time, but...) :
- excellent macro system (even if mzscheme seems to have a "dirty" defmacro too),
- the empty list is the false boolean,
- the syntax (particularily  and :) is really a great improvement,
- I never built webapps that fast (and with so much fun),
- my list is close to almkglor's, but he's before me in the leaders' list, and I'll never catch him, so I can't follow him on this point :)
Now, sure, PLT is an excellent language/environment. But it's waaaaay older, that helps. Actually, if today I was given the opportunity to program in a Lisp of my choice (for work I mean, not for fun), I'd use Arc for a basic webapp and mzscheme for almost everything else.