Karthus is more than a walking ultimate. Don't underestimate him.
Yes, you annoy the fuck out of him by denying him global executions with that
.
That does not mean that he is no longer a threat.
If your cockblocker dies before (or even slightly after) Karthus does, it doesn't mean shit.
If Karthus can wade through your team and kill you with basic abilities, he doesn't NEED to use his ultimate.
If you stand on his fucking corpse and underestimate it, I cannot begin to describe the level of fucked you are.
I get it. The running joke about him and his "Just Press R" playstlye. If you want to play around just that, fine. Just don't complain when you and your team die to the onslaught of explosive skittles because you "wanted to save your shield for his ult".
He is the fucking lich king. Don't underestimate your heavy metal overlords.