This seems like a question that means nothing in the abstract. If I'm playing against 1 tank 3 ranged and my team has a ton of frontline damage (i.e. Tychus, Leoric, Kharazim), then my entire team wants to blow up their tank and that should be the gameplan in fights, right? At that point, my damage into their tank isn't "wasted". Meanwhile, if my team has multiple backline divers, I might want to fight past their frontline and pump damage into the dive target. If my team has powerful displacement/isolation tools, I probably wait until we displace/isolate someone and followup with damage and root (e.g. Artanis swap into root + D is almost a guaranteed kill).
In general, I think with the Genji/Maiev/Kerrigan type, your goal should be more to CC them down than to worry about your own damage onto that hero. If you can get the double auto attack with D to followup your own CC on the enemy diver, that's great! But it's more important that you just land the root in the first place.