Spoilers.
Yeah, I liked how the letter was used. I don't like how some other quotes were used, though. Like, "This time, I'll fight." I imagined it said much more deeply in a different scenario, not like it was with Kairi running with a Keyblade. And "We'll go together"? I thought it meant Riku and someone else searching for Sora or something, but it was used at the end for the door to the light.
I'm not disappointed, it was just used differently than I expected...
EDIT: Yes, it was Kairi's connection to Sora's heart. I know some of you dislike Sora x Kairiness, but it's hinted throughout KH I, II, and CoM (remember when Namine said "There's another girl, and she's your light."?) that Sora loves Kairi, and that was pretty much almost guarenteed when the letter arrived (Riku knew it was for Sora) and when they did that slow motion close up of Kairi in the ending as Sora swims towards her (right before Donald and Goofy tackle-hug Sora XD) with her hair blowing in the wind. Total cliche XD