A few things:
1. This is good for scouting
All scouts admit it is much harder to scout high school games, which means you draft on potential, or unknown potential, since there's a lot more talent dilution. We don't want teams having draft busts. money going to over-valued players = bad for the league.
2. This is (somewhat) good for injuries.
Granted, for those who are naturally blessed like Lebron, Dwight or Amare, this doesn't matter, but for most players, if you look at their senior year high school film vs their college freshmen year highlight tapes, the biggest difference is their physical size. Carter comes to mind, though it's somewhat true for Pierce too. Weights and getting into physical shape has much more of an emphasis on it in college than in high school, so it reduces the likely hood of injury. Guys like Sean Livingston would've been helped a lot by this. This is somewhat minor, but it does help.
I know the main argument, that stems from a lot of people that because kids can get injured, they should go to college and "make their money now" because it's much harder in the future, and they could blow it by getting injured in college. However, playing in the NBA is a privilege, and not a right. NCAA (as crappy as they are) do try to put as many measurements in place to make sure these kids don't get injured. The big picture however, is that it is better for the league to draft better players, make slightly better decisions, and have less wastage.
We can keep giving examples of one and done busts v high school busts, though sample size really isn't big enough. If the scouts say it makes an easier job for them, I mean, whose to disagree?