Right, but Skaar is also protecting Banner. Remember, he doesn't want anyone taking Banner out before he can get his shot at the Hulk. So you'd need to get through all of Banner's gadgets and his son, who's almost as strong as the Hulk, has the Oldpower as well, and is much smarter and combat-savvy than the Hulk to boot.
Osborn may be crazy, but his plan's sound. If he turns Banner into the Hulk again, he not only doesn't have to deal with Banner's gadgets or Skaar, he actually gets to sit back and just watch Skaar go to work on the Hulk himself. What does Osborn really care if Skaar can't take him? Either Skaar will kill the Hulk or the Hulk will kill or at least beat the crap out of Skaar. It's a win/win for Osborn. Then Osborn can just come in and mop up whomever's left over--if it's Skaar, he can sic Daken on him again. If it's the Hulk, he can wait and take him out as Banner with an orbital satellite or something.
Accelerating Banner's return to the Hulk was pretty clever, I think. I mean, it'll obviously blow up in Osborn's face horribly, but on the surface it's a really good idea from Osborn's perspective.