I thought Murray was great in MI3, I really did.
And I think that's the way it should have remained. Bringing him back was a bad move and a bad idea.
I don't think that a character being nice and original means you need to bring him back.
I'm not hoping for old characters to keep popping up in ToMI. A reference from time to time is nice, but what I'd really like is some new, interesting, original characters.
The force of Murray was that he wasn't some old character coming back. He was new.
Sure some charracters are reccuring in every game so far (Guybrush, Elaine, Lechuck, voodoo Lady, Stan...) but the new characters are what makes every one of the games unique in its own way.
I love human Lechuck, I'm starting to like Windlow and I'm pretty curious about Morgan. I'd rather see these being more developped than Largo, Wally and Murray pop back for no reason whatsoever.
(Of course one of them is always welcome to come back if there is a very good reason for it).
I don't think Murray is overrated as a character. I think fans who like something tend to ask for more of it (be it a show, a series of books or a character) and forget that one thing that made them like it in the first place is that it was new and original.