Does one NEED floor standing speakers for HT only?

"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.

Swapping out my B&W 601S2s for a set of Thiel CS2.4SEs, the biggest difference was in movie soundtracks. Movie scores can be epic.. grand, large in scale and generally benefit from a good stereo speaker.

In terms of frequency responses, a good AV receiver's bass management should be able to send the low frequencies from the mains to the sub, if you set the speakers to small, so theoretically, you aren't losing anything.

But it makes the main speakers sound, well, small :)

Putting them to full range IMHO gives movies with orchestral pieces the scale and grandeur they deserve.

O Yeah. The one need floor standing speakers for HT
Also the wall and ceiling speakers would be good idea.