And if they contributed to the story? If they were actually a part of it?
No. It's usually a bad idea to bring someone back when there's no reason too (and so far ToMI story doesn't show at least a single reason to bring the trio back), especially on fans' demand. It's not the matter of contribution to the story, but of the story's logic.
Stan kinda became a running gag in the games, just as "I'm selling these fine leather jackets" and things like that, something that's in every MI game, so I don't really have anything against him (and with him it doesn't really matter how, where, or why he started some new business, he was making laminated business cards in a coffin, for crying out loud). More than that, I will be surprised (though not frustrated) if he will be left out of the game. But I doubt barber trio would do any good. Heck, I'm not even sure that Murray would be in his place (and, let's face it, although I did enjoy EfMI pretty much, and loved dialoge with Murray in that game, he WAS out of place there. Heck, he was out of place in the second half of CMI). So, it's kinda a hard thing to think about, actually...
I WOULD like to see Herman Toothrot/H.T. Marley and Marley Family Lawyers in Trial and Execution of Guybrush Threepwood, though. I mean, H.T. being Elaine's grandfather and Guybrush being a part of Marley family, it would be logical for them to appear. Besides, the Lawyers have yet unopened potential (while barbers don't really have much of it left, considering how much time we've spent with them and little with those lawyers), and I love their music theme from EfMI.