Would group write work with some control panels, or are the users just so unrelated that it needs to be world writable?
A very common case Plesk end users run into is using mod_php (PHP running as user 'apache') with a web application which needs to be able to upload files (file uploads for a WordPress installation for instance). This won't work out of the box since the upload directory will be 755 and owned by the domain user and 'psacln' group by default, which does not allow user 'apache' to write files to this directory. User 'apache' isn't a member of the 'psacln' group, so setting 775 permissions won't help either.
Since end users can't change the user or group owner they have no choice but to change permissions and setting the upload directory to be world writable is the only way user 'apache' will be allowed to write files to the upload directory. AFAIK using 777 permissions is actually the only way PHP file uploads with mod_php will work in Plesk's setup without an administrator stepping in to change user or group ownership, use sticky bits or some other voodoo end users are not allowed to perform.
When a domain is running PHP via FastCGI it will work out of the box, since in that case the PHP code will be executed by the user owning the PHP file.
The WordPress file permissions documentation at http://codex.wordpress.org/Changing_File_Permissions
tries its best to explain how file permissions work and why 777 is bad, but my experience is that end users feel the documentation is way too complex for them to understand, so they'll find some forum post which tells them using 777 permissions (recursively!) will solve all their problems. They try it and ta-da, it works! Some even go as far as recursively setting 777 on all files and directories in their httpdocs directory for good measure.