Well, here's why I like it:
-The grid system forced me to prioritize. It meant everything affected each other -- keyblades, magic, abilities, even items, all took up space that influenced what you could equip. Is that a bad thing? To me, not at all. It meant everything had a tradeoff, and I liked working within the limitations it gave me. Because you needed to closely manage everything, everything became important. I took it as a challenge to decide what I kept and what had to go. It meant I needed to adjust my playing accordingly. Don't have as many potions because of your new keyblade and magic? Welp, guess you gotta be careful until you get more panels!
-I liked the limited magic. In fact, it's my favorite system besides BBS's. I barely used magic in other KH games besides BBS, so Days was a pleasant surprise by making it both a limited resource, and actually effective for damage. And certain spells are only effective against certain enemies? Uh... yeah. Either stock the spell, make use of the spells you do have, or find another way to kill 'em.
-Oh, and I never needed to go out of my way to use a limit break. I found plenty of ways to clear the screen without them, not that I used them much to begin with, because I bothered to manage my HP.
-The story? An unnecessary installment isn't synonymous with an unwelcome installment; I didn't need Crisis Core to better appreciate FFVII, but I like it on its own merits and am ultimately thankful that it does exist. Days gave me a game about Roxas's time in the Organization, and I like Roxas, sooo... cool beans? I liked following it, endless clocktower scenes and all. In fact, I dislike the 1.5 movie specifically because it removed/altered many of the small-but-poignant scenes that actually seemed to flesh out the characters. I also appreciated how Days directly connects to multiple games. It's a treat to have known what went on in the background of the other installments.
-I didn't find the missions repetitive at all, at least not repetitive in the bad way. So what if most missions boiled down to "kill all enemies"? It was still fun to kill the enemies, with my prioritized abilities (see my first point). Mobs had enough different placements and strengths to shake things up... though the less I say about Wonderland, the better. (I didn't like it in KH1, either.)
If you're going to "refute" me by saying that, no, I was incorrect to have fun for my listed reasons, then I'm curious to know how your arguments will magically poof away the fun I did have playing it... multiple times. Yes, it's the KH game I replayed the most out of all of them. Still love it. Or, if you're gonna make fun of my bad taste, remember that the objective of playing a game is to have fun. I had fun.