I think Season 1 of TNG is probably a better window into Phase II, at least from a potential story perspective. Decker, Ilia, Xon and elder statesmen Kirk are all there, just with the serial numbers filed off.I admire TMP more for what it's trying to be, rather than what it actually is. I like that it doesn't exactly fit with what came before or what comes after. It's a window into what Star Trek: Phase II might have looked like, so it's kind of worth it for me on that basis alone. It's a curio for sure, but it's the only time we get to see Roddenberry's idea of Star Trek unfiltered on a cinema screen.
For better or worse, in the whole of Star Trek I think there's nothing else quite like it.
I agree with you that it's not the best of them, but there's a lot to see in there that's never seen again.
I do 100% agree that TMP doesn’t really fit what came before or afterwards. It’s the closest Star Trek ever came to being “hard” science fiction. Aside from the basic TOS designs of Enterprise and Klingon ships, almost everything else feels like 70s futurist/NASA designs; and it’s apparent more effort was made to try and work out how things actually worked in universe.