See, the entire reason I play KH is for the "filler." The overarching plot is there because Tetsuya Nomura is incapable of making something that's good all the way through. I accept that, but the idea of caring about Riku and Sora is completely alien to me.
Riku is somewhat more interesting in KHCoM. Basically the entire story is him being pushed around by Ansem, and the Orgy members, until he basically says "fuck it, you guys suck, I'm just gonna kick your ass.' Unlike Sora, he has a legit reason for going through this shit; he did kind of get eaten by a bunch of monsters, has no clue what's going on, and his only ally is not really with him physically, so he's kind of alone on this endeavor. He also has to face the guy whose been mind raping him since KH1, so you almost feel bad for him...then he gets his act together and the "almost sympathy" transforms into "mild respect" as now Riku's just giving each villain the finger, verbally owning them, and THEN kicks their asses.
Contrast to Sora who just whines about his memories, villains mock him, pulls one of those "I don't believe you! Quit mocking me!!!" and then finally fights. Sora's even told he's being manipulated and he still...gets manipulated. Riku's more like Ramza; he's being manipulated, he knows that's the case, but frankly, he doesn't really have much of a choice but to go along with it, and hope for a good opportunity.
...you know, I'm reminded about another reason why Riku Mode >>> Sora Mode.
When Riku runs out of cards? Select Reshuffle, OH LOOK FULL DECK! That took all of 1 second!
When Sora's out of cards? Sits there and charges...ok, not bad. ...wait, now I have to sit double the length when I run out? ...now TRIPLE!?
Yeah, why Sora had to keep going up (thankfully it caps at 3) I don't know. Oh, Riku's ability to use all monster/boss cards whenever he wants was a nice boon; those things really weren't so overpowered, but at the same time, were really handy.
In any event, I generally agree that you don't play KH for plot, and that the "filler" is more interesting, though, the War sequence in KH2 was generally well done, but that's mostly a style thing. 1000 Heartless Battle (with good music alongside it) + the FF Cameos actually DOING SOMETHING (this includes the Pseudo-Temp forms they have) + Stitch shooting some Heartless + Failure Emo Cloud getting verbally owned by Sephiroth + Mickey Mouse + Tron. YEah, fun sequence in general.