If you do not think that Greene is a starting-quality small forward, then you may have an unrealistic concept of what a starting-quality small forward is. Most starting small forwards are not the stars of their teams; most starting small forwards are either spot-up shooters (like Galinari and Richardson), or defensive specialists (like Prince and Battier). Barely a third of the starting small forwards in the league are averaging even fifteen points a game, and only five are averaging twenty or better. In fact, most starting small forwards in the league appear to be averaging between eight and twelve points, so he appears to be in good company offensively. Defensively, we see this kid improving on a daily basis, and he's certainly on par with the average 3. His rebounding isn't quite up to snuff, but I'll allow that from a second-year player who not only hasn't yet been given a clear role on the team, but hasn't even been given a clear position.
The LeBron James', Carmelo Anthonys and Kevin Durants of the world are few and far between, but just because Greene is not on their level does not mean he's not a starter. This seems like the same line of faulty reasoning that causes people to think that, because Jason Thompson isn't as good as Chris Bosh or Carlos Boozer, that he's not good enough to start. I'd be willing to bet that there are as many as five playoff teams that would, right now, trade us their starting small forward straight up (and, in a couple of cases, maybe even throw in a draft pick) for Greene.