I don't understand where the opposition to the NBA age limit is coming from. As far as I can tell, the case for the age limit is a slam dunk (no pun intended).
The primary purpose of the draft is to give bad teams a chance to become competitive. Ideally, the draft would allow them to become competitive a) quickly and b) reliably. Drafting high schoolers detracts from both of these goals. The occasional LeBron aside, high schoolers are generally unable to contribute right away. This means that they sit on the bench for years while the team continues to suck. As for reliability, since these kids have never seen anything beyond high-school competition, and since their bodies haven't fully matured, it's naturally harder to tell if they'll be good. Thus, allowing high-schoolers in the draft increases the likelihood that bad teams will be saddled with someone like Kwame Brown -- who helps them neither immediately, nor ever.
A stupid response to the above is something like the following: "if you want to win right away, nobody's making you take the high schooler". The fact is, teams are trying to do what's best for them over the long run; right now, this means taking a gamble on a high schooler because failing to do so means you might be letting the next Kobe or T-Mac go. Anyway, there generally isn't anyone coming out of college who can fundamentally improve your team. We shouldn't force teams to choose between mediocre college players who will help a little in the short term, and high schoolers who might help a lot in the long term!
Another stupid argument that I've heard a lot is that it's a double standard (or even racist!) to have an age limit in basketball when there isn't one in tennis or golf. But sports which don't have a draft that works like the NBA's do not have to deal with the problems described above. And since those problems are the biggest impetus for the age limit, the golf and tennis comparisons are entirely irrelevant.
Another purpose of the draft is to generate publicity for the NBA, create excitement among the fans and get high ratings. Again, unknown high school players hurt this cause, while college players bring built-in fan bases into the league. Plus, rivalries that develop in college can carry on into the pros and spice things up a bit. (Also, nobody disputes that this would be good for the college game, and all of its fans... This is a side benefit that doesn't get the attention it deserves.)
And what are the drawbacks? The only one I can see is that a couple of kids a year have to wait another year or two before becoming millionaires. I certainly don't begrudge them trying to get this money; I would, too, in their position. However, I don't understand why they're entitled to it, if allowing them in the draft hurts the game. Finally, it's not clear to me why the age limit is considered "anti-player", since the high-schoolers must take a veteran's roster spot when they come into the league (and the veteran is about as likely to be black, so don't start with the racism stuff). Often, they don't even play, they just sit there waiting for their potential to kick in. Why is such a high schooler is more deserving or meritorious than a veteran?
I have another, subtler complaint about all the hand-wringing about the poor black teenagers who are getting screwed over. Understand that this is a tiny handful of kids, who are most likely destined for vast wealth anyway. The emotional power of the argument derives from a subtle association of these few with black teenagers in general; to hurt them, the argument suggests, is to hurt poor African-American youths. This, of course, is just the other side of the dangerous delusion, unfortunately shared by too many teenagers, that they have a realistic shot to make it in the NBA. This belief is pernicious and needs to be challenged.
Now, in this post I've been defending some vague, ideal age limit. I'm not sure that the one that was actually put in place will make much of a difference -- it seems too weak (though of course that also mitigates any possible harms of it). Nevertheless, it seems like at least a step in the right direction.