It's a good idea, and suggestion.. But I doubt this will be taking as a serious note..
.. I don't think many developers wants to do this... And the easiest place to have a plugin save it's data is in the same folder as the program is installed.. That's pure sense...
IMHO (But this is probably beacuse i'm running without user accounts and the likes) I hate when other programs put loads of shit into my documents and settings, and Application Data... I just HATE it... I don't want random programs to store random things a place like that.. If I decide to install my program to my d drive because i'm in lack of harddrive space on my c drive (or any other reason), then this program is supposed to stay and only use space on my d drive... No where else... This is just my view and probably only because of the way I use my computer and orginise my shit on my hard drives...
Though, I see the problem that plugins migt not be able to write to the "Program Files" dir... Though why don't you just install messenger plus a place where the users have access to write to?... Messenger Plus have an option to change installation path... And all plugins makes sure they are installed in this path and saves their data their.. This kinda eliminates your problem...
Anyways.. Your suggestion is still good and developers should really listen to it, this is the way WinXP is build... But then again, we would then have to make extra code to be backwards compatible with the older windows, since they don't have the same kinds of Application Data folder...
This was not ment to be rant or anything.. Was just that coders have to think about the consequences before they act...