Well we know that Starfleet has noncoms. They are the guys with no rank insignia on their collars. I could be mistaken, but I think its been said that O'Brian could become an officer if he wanted too, so a program to allow enlisted to become officers exist. So if you just want to get out there now you can become an officer later.
I don't subscribe to the belief of ROTC programs for Starfleet officers. I don't know why really, I just don't feel it is that way. However I'm willing to believe that Starfleet has several training facilities across the Federation for enlisted personnel (although even this I doubt).
And as for reasons why someone would enlist, on the episode of Voyager where Janeway decides to work with the three worst crewmembers, one of them only enlisted in Starfleet because the school he wanted to go to required time served in deep space. So many reasons exist.