Rubio has flaws and ideologically, is below some of the other candidates, there is no question. He is though, on balance, a better option than we often get. Better than Romney or Mccain. He also polls better among young voters than our other candidates except Paul (who while polling well among some demographics, doesn't turn that into votes).
To the average joe who really doesn't think about establishment versus others, as the polls reflect, they like Rubio. Most voters aren't as informed or detail oriented as folks here and don't keep lists of who is in or working with the establishment that day. They see what he says directly to them in debates, commercials, and online. Rubio is very good at talking to people on the other side of the camera directly, something our better candidate does have a challenge with. He also doesn't insult the voters or make them cringe (as does one of our other leading candidates).
He has a history that shows, while not perfect, he can be at least be put as an 85% Conservative, and he isn't a johnny come lately to that.
He wouldn't be my first choice but I wouldn't have a problem pressing the button for him if it came down to ti.