Does one NEED floor standing speakers for HT only?

Baranowski, Good sub is expensive (even if you already have it you could sell it). Same go for the stands. Integration of the sub is not always the easiest thing. I decided to settle for floor standers with 30Hz response and no sub. Certain powered subs, allow with microphone, to compensate overall response which might be helpful if you have room issues.

"MIDS are what the floors would be bringing more to the table"

That might be true, but for HT you should have center channel that usually has excellent mids. If you already decided to have sub (for special effects), then it might be better to invest in good center for the price difference between monitors and floor standers. Center channel in HT improves imaging for off center seats (widens sweet spot). Same goes for two subs - more even bass at different seating locations.