I wouldn't recommend Slime to ANYONE because the developers do not care about security. The issue[0] of a gaping security hole has been open for going on two years.
That actually scares me enough to disable Swank on my laptop. Actually, to disable all localhost services that could by any stretch of the imagination execute code.
Tl;dr: web sites can send requests to localhost TCP sockets despite origin restrictions using a trick called DNS rebinding.