Whats Pub_FTP for? Is that for when you let users upload pictures to put in your forum for instance?
Won't even work for that because the pictures have to be in public_html to be viewable; in all honesty I have no idea what public_ftp is good for ON X10 at least.
My -guess- is it's a leftover from Apache for places like a video game patch repository - you'd have anonymous login's enabled and people could just FTP in and snag 200mb-Patch-12-31-08.zip for instance.
On x10 though, only the account holder's actually supposed to be doing anything on the account, so a public_ftp wouldn't quiiiite work like it's supposed to. I think it's just a left-over.