The one thing that Judoka (those who practice Judo) have (other than a different view of the clinch) is that in a Judo match once somebody is thrown, the two Judoka have a very limited amount of time to lock in a submission before the ref stands the two up. Apply that to MMA and once they get somebody on the ground, they have explosiveness that can get a submission.
Of course, in the MMA world, I don't think that anybody trains in only Judo without at least a little BJJ. The reason behind this is that (to my knowledge) you're not on your back at all, and in MMA you're on your back a lot and if you just trained in Judo you'd be screwed.
It's very hard to survive in MMA doing just 1 grappling art (whether it be Judo, BJJ, Sambo, Wrestling, etc.) because not all of the arts have every situation, that's why Submission Wrestling is a good choice for the wanna-be MMA fighter.