This is pretty much my biggest problem as well. Yeah, I most likely would have bought the AC costume for Noel and the other costume for Serah, provided that they actually released them during the game's launch. I'm really not comprehending SE's strategy here because by the next month, most people are going to be done with the game (and the very incremental DLC thus far isn't going to keep people occupied for more than 10-30 minutes).
I guess I'll repeat my thoughts from before. From my understanding, there IS a reason SE is doing this. They're trying to do two things. Increase the longevity of the game and prevent the games being sold as used games. I'll explain the first.
Essentially, they release costumes because it adds a new visual aesthetic. SE thinks that you will go back to play through the game or the new "story" content if they release costumes steadily. While you, and me included, might not fall under the audience, I know that some people are psyched that these are coming out and are willing to buy them and play the game again.
Secondly, by constantly streaming out DLC SE prevents Used Game sales in a way. This ties to the first reason in a way. SE is essentially preventing people from trading in their game because there is content coming out and, once again you might not fall under this category, people will keep the game because content exists and is coming out steadily.
These are just my thoughts on SE's strategy. While I can't say this doesn't work or not, I personally say that it's crap strategy when the content is bad and the price is just as bad.