Win NT (any version) processes are quite expensive

Windows doesn't even have forks, so the point is moot anyway. Notice how the Perl 5.6 thread model was largely an attempt to emulate fork() for platforms which don't have it (even if that wasn't its stated goal, it certainly made that impression).

In sharing data between processes, you presumably control everything that happens between the two processes.

"Presumably" being the keyword, because this is about the effect of a) bugs and b) security holes. With threads, both occurences can kill off your entire application. With forked processes, they can only affect the child in question except where a resource is explicitly shared.