I agree that if MGM never tries to build their own parks again (or enter into a full blown partnership), then they don't have much to lose by letting Disney use the name, provided of course that Disney maintains quality that MGM is happy with.
That said, I don't think they gain that much though either. Disney is pretty much the only major movie studio name that has anything close to a distinctive brand identity. Sure, everybody has heard of Paramount, Universal, MGM, 20th Century Fox, etc, but do people really differentiate between them when it comes to new product? If the general public hears "Its a Universal movie", or "Its a Paramount movie", does it make a difference? I don't think so, so they don't gain that much by just having their name attached to a Disney park. A few more people might think of MGM more often, but it doesn't make them any more likely to go see an MGM film.
Now, if they were to really work the tie-ins for classic MGM films, to the exclusion of other studios' films, then maybe. (But I'm not saying that's what Disney should be doing)
So I can see MGM not really caring about it one way or the other, unless they decide to build their own parks, or if Disney really ponies up some real cash.