I'm not so sure about G.fast, but the vectoring engine might be under more stress if the signal levels for different lines keep changing, depending on if they've got any data being sent or not. With VDSL2, what would happen is that when one line disconnects or re-trains, then the vectoring engine would have to re-calculate the crosstalk cancellation for all the remaining lines. I thought that was why a low power mode wasn't particularly wanted for VDSL2, because it would make the crosstalk levels more changeable.
With an always on VDSL2 line, the amount of crosstalk it causes should be fairly constant, so it would be easier for the vectoring engine if it stays constant.