You want my opinion on this? Well uh, sure it is perfectly logical for waterbenders to make steam, or in this case mist/fog. I've recently found out this thing called ice7 (or ice seven) where it's not "cold" ice like others would think, but it's water that is so heavily dense that it turns into ice. The opposite thing could be said with mist, instead of changing the temperature of water (like many people think), they could be simply changing the density.
(All of the above is just speculation, mostly at least.)
But for a sub element, I believe it'd prove for nothing TOO useful, nor anything we've actually seen in action many times in the show. But I guess there'd be someway to use it. Allow me to narrow down a few options
Can we blow things away with mist? No
Can we harm things with mist? No
Can it be used in a sort of evasion? Yes
Does it have too many uses? No
Can we reuse steam for water? Maybe?
My final thought on any way to use mist, would be to turn water into floating particles that can be utilized as a water source that is suspended in the air, enabling for potential combos and extra ways to use water. And on terms of more ways to use mist, it could be very possible for waterbenders to use it to easily put out fires by smothering it with moisture.
