But Haruhi doesn't walk all over Koizumi, only Kyon, Mikuru and occasionally Yuki. I mean, he can't "not let her" if she didn't do it in the first place, right? The only time I can remember seeing Haruhi mistreating him was in Plotting, and that was to give him a Valentine's Day surprise. She never forces him to do anything. At first I thought he was forced to carry things like Kyon, but in the last volume that came out, he didn't help carry stuff because he was busying hugging his new game board, and he got away fine. So I suppose nobody ever forces him to do anything in the SOS Dan, he just volunteers. Also, I do remember him stopping Haruhi when she was going overboard [novel only], but I don't remember where/when. I do remember that it only took few words.
It's also worth to note that Koizumi's true personality has nothing to do with his persona. Many times in the novel, he hints that he wants to quit and live a normal life. When he warns the Council President to not let the persona take over his true self, he was also reminding himself that he wasn't what he appears to be.
In one chapter, the Never Ending Summer, he mentioned what he liked. Just like Haruhi and Kyon, he used to dream about all the extraordinary things, like aliens and espers. His hobby was actually stargazing. That surprised me, because I have always believed that his hobby was stalking (sorry!). So his true self must be very alike to Haruhi's, just more rational. He's a dreamer just Haruhi. He wants attention from special people.
The problem right now is that the own SOS Dan treats Koizumi like air. Kyon seems to notice him more and more, but it's more pity than anything else.
What Haruhi and Koizumi have in commun is that, while screaming about aliens and espers and time travellers, they are actually seeking someone who would care about them. Haruhi believes this person is Kyon, because he's the first to turn back and talk to her, not to flirt with her because she's pretty, but to know what she thinks and desires. That's why Koizumi is willing to let Kyon have Haruhi. It's not because he's a counterweight, it's because he cares. He doesn't do this just to avoid the End of the World, he really cares about Haruhi.
When Koizumi decides to let Kyon have her, in my opinion, he has just made him more worthy of Haruhi's love because he proves that he cares eve more. But she barely notice his presence.
I wish one day he would stop being Haruhi's weather man and show her his real self.