I generally agree with you.
I have all the expanded and unreleased stuff from those two albums on a Columbia collection called "Nostalgia in Times Square." I certainly like having and hearing the full length stuff.
The thing is, though, that Mingus went into those sessions knowing that he would record more than he would use. The original albums reflected his conception of how they should sound. The expanded "Mingus Ah Um," which I also have, simply doesn't have the same pacing as the original. I'm glad the full length performances are out there, but I feel they should supplement Mingus's original albums rather than replace them, IMHO.