I don't think it is useless at all. I got attacked by two hitsquads, one thalmor and one vampiric. Sandwiched in the middle, I couldn't believe my luck when a legendary dragon started attacking. I used bend will, fried up some elven and necro butts, and went my merry way. It is not useless, just because it can't be used to explore. It is like an upgraded version of calling odahviing since you can control who what and when it attacks.
Actually you kinda proved my point there. (No offense)
Really you need a dragon nearby in order for it to be useful. You said "I couldn't believe my luck when a legendary dragon started attacking. I used bend will". In that statement you kinda proved my point that you have to be lucky in order to have a dragon nearby in order to make any use of this new mechanic.
Sure you can get on a dragon and fast travel. But hay the place you gotta go to probably has not been discovered.
So over all if you cannot get that dragon to be where you need to be. It's useless most of the time. Sure it has a few moments In Terms of combat. But most of us did not get the dlc for the dragon riding combat. We got it to fly around tamriel instead of using the fast travel.
So although it is not completely useless. It is very close to it.
In comparison to what it could have been........I don't even want to go there in terms of usefulness.