Judo would have a comparable function in MMA as (Greco-Roman) wrestling, yet it is slightly different. The training with a Gi has two sides to it. On the one side you are used to throw your opponent with the help of his Gi, which makes it easier to throw him, so it will be harder in MMA to throw people who don't have a Gi and Greco-Roman wrestling has an advantage of being used to throw people without a Gi. But then on the other side the Gi training will have an positive effect on you take down defense. You are used to defend against take downs where you opponent is helped by your Gi, so you develop an even stronger base.
And as I said in other threads, I believe that Judo will be the next style to be more appreciated in the next 3-5 years of MMA. At least in particular by BJJ based fighters as Judo is the origin of BJJ and therefor technically the most natural link between stand up and ground fighting for BJJ fighters. It is easier to transition from a Judo throw to BJJ submission grappling than from a wrestling take down (in particular collegiate) which brings you rather in a ground and pound position. And last but not least, Judo is probably much bigger in Brasil, where most BJJ fighters come from, than wrestling (as they don't have that collegiate wrestling culture like in the US), so it's also just easier for them to find good Judo partners for training than wrestlers.