So maybe there really needs to be a UI distinction between desk accessories and other things, just to make it easy to call them up at any time.

It makes sense to put them on a separate plane of the desktop interface from full-fledged application windows, I'll give it that. But I don't see the logic behind making users learn two different ways of interacting with program objects, or making developers learn two different ways of developing them. Anything that's good practice for a standard application should also be good practice for a lightweight app-let.