The benefit will depend on the sub's internal amp. I believe that the point of "auto on" was to decrease power consumption and internal heating. Depending on the amp, the standby power in "always on" might be nearly as low as it is in "auto on," so there might not be a significant benefit.
I noticed that my SVS is as warm to the touch in either mode when it's not being used, so I just leave it set to be always on. My previous sub, a B&W, was significantly warmer when always on, so I set it to "auto on."