I doubt you'd derive any soil heating benefits from floating row covers. It's open weave, while it will retain some warmer air, just isn't geared for soil warming and its white color defeats the purpose by reflecting light. Plain black plastic should be avoided, but black permeable landscape fabric works very well for warming the soil.

Dave

Do you think floating row covers are useful for early crops? I read they can sort of act like greenhouses and trap in heat. Is it just that they keep the air warm underneith?

The maximum soil heating advantage from any black material will be had if it is in contact with the soil, not above it. Make as much soil contact as is possible, it's a thermodynamic thing.

Do you think floating row covers are useful for early crops? .

Yes, they can be as a tunnel over or drape cover on chilly nights by trapping a bit of warmer air inside. Depending on the thickness used you can get 7-10 degrees of protection for early crops if there is no wind. But don't count on them for a hard freeze.

But they doesn't warm the soil. That is a different issue. ;)

Dave

Research at some universities has indicated that Floating Row Covers might increase the soil temperture under them by 1 to 3 degrees, but the greatest benefit is that they offer some frost protection keeping the plants growing under those FRCs 2 to 4 degrees warmer than plants without.
All the things you listed are frost hardy and do not need the protection these might offer except possibly for insects and other critters.