I was gonna make a new suggestion thread, but seeing as this is here:
I agree it should be implemented based on the fact that level differential effects your ability to thief effectively now. If you donate for an amnesia brace you can prevent leveling and having to make new thieves once yours starts breaking items.
So for pros we have:
Encourages donation
Potentially saves TMs
More items potentially farmed, encouraging trade between players while helping keep the value of farm-able items such as mushrooms and star pieces stable.
Implementing this as a wild encounter effect only wouldn't hurt the comp aspect of the move as Atuan suggests.
[spoiler]Dont bitch about me necroing a thread, you wouldve told me to "learn to search" otherwise, fgts. [/spoiler]