if they worked in XP why change things
Because they don't work in XP. Once you have another user sharing the same machine on mirc-pre-6.3 you'll find that you'll be forced to share the settings for every user account on the system. That's broken. Owners of internet cafe's or kiosks have come in here relatively often asking how to get around this.
Making something "work like XP" is only considered "fixing" it if it worked better on that OS than Vista. In your case it may not work any better (that doesn't mean it's any worse), but for many, the AppData changes improved the program.
Finally, if you upgrade a pre-Vista version of mIRC (before AppData compatibility), you'll find that the new mIRC will work the same way it did before. The only change in behaviour is for *new* installs, so most users will never notice a difference-- in fact, most users nowadays should expect application data to live in AppData (or Documents), not in the Program Files directory.