I for one am jealous of states like AZ and UT that allow ATV's/UTV's to be licensed for street use provided they meet certain criteria. Colorado allows me to do so for my motorcycle, which is a plated dirt bike, but they won't allow ATV's or UTV's to convert to a street legal title even if they meet all the DMV requirements. This makes no sense to me. I generally don't see how driving a properly equipped vehicle of any kind on the street harms anyone, aside from the operator perhaps, regardless of the vehicle's original designation. In my mind, it simply provides us with greater freedom, whether we choose to take advantage of that freedom or not.
As for the park service allowing UTV's on roads or trails that allow other licensed vehicles, it makes sense to me. I never understood why a non-plated dirtbike was forbidden to ride the White Rim, while the same bike with a plate was fine. It's not like turn signals are needed anywhere on the White Rim... Though I'd rather not have the loud piped UTV's blatting around in my national parks, the bro's with their diesel trucks and 6" exhaust pipes are already there, and I find most UTV's less obnoxious than bro-dozers.
For places requiring permits, they're not going to allow more permits, the additional vehicles will just compete for existing permits. For those that don't, I doubt there will be any real influx of traffic simply because it's not terribly fun to drive a UTV a great distance to get to many of those places.
I can't believe I just defended the one vehicle type that I generally despise... I must be off today. Or perhaps I'm just less off than normal...