In MMA competitions when they go to the ground it's nice clean padded canvas, no one goes to the gorund in the street if they can help it, broken glass, vomit, dog mess, other peoples feet within kicking distance etc.
If an MMA fighter is going to fight on the street I very much doubt any rules will come into his head, as everyone who has ever sparred in a club will testify, competitions are very different from a real fight, it won't be MMA it will be Vale tudo - anything goes!
I know it's different in the States but most MMA fighters I know have trained in TMAs before, some still do. I think the advantage MMAers have is that because of the nature of their training they aren't fixed as to being only able to do standup or only being able to do groundwork, they expect anything as well that can come their way whether it's kick, punches or takedowns.
I would suggest watching Bas Ruttens excellent dvd on self defence.