A lister on the AppleScript mailing list writes:
The Scripting Interface Guidelines agree with you: make and delete should be used to create/destroy objects completely, while add and remove should be used to add/remove existing objects to sub-groups. e.g. Compare Address Book, which uses add and remove to add/remove existing person objects to group objects. iTunes predates the SIG, however, and while its scripting interface is fairly extensive it does suffer a number of longstanding design quirks.
Yep. I'd be happy if they just made it like it used to be.