I disagree, 358/2 Days wasn't filler (if anything, KH3D will be, since Nintendo asked for a KH game on the console).
However, here's my theory on why there wasn't a FM for Days. Its simple really, it was more like Chain of Memories, it wasn't one of the main games. One could argue that Birth by Sleep wasn't either, but Nomura said it was just as important as a main game, saying to think of it as more of a Kingdom Hearts Zero then a side game like CoM or Days, but it couldn't be named like a numbered title because a numbered title must have Sora as the main character. As with CoM & Days, Coded/reCoded probably won't be getting FMs either. Again, one could say that reCoM & reCoded are FMs, but their really more like remakes, hence the 're' instead of 'Final Mix'.