It's harassment and destructive to your team's morale. (There's a loading screen tip that mentions negative attitudes decrease winrates)
Sure, in this case he was AFK and couldn't read it so it can't bother him, he/she may have actually had a good reason to go AFK (some people are complaining about struggling to connect with the server move)
About 55% of my games have people saying to report me, either because I'm playing bad or because they're playing bad and need a scapegoat. It's distracting, adds unnecessary pressure when I may already be feeling pressured by my opponent or game not going well, offensive to me as someone just trying to enjoy a game and makes it hard to work as a team when I feel like I'm playing against my own teammate.
I usually report it as harassment. Good on RIOT for taking action against this.
Funny time I once countertrolled, I was being harassed by a Teemo player from game start so I let the Ziggs in his lane feed off me 9 times so that he could take care of Teemo for me, upon failing that I went on to kill Ziggs over and over, how I managed to make a comeback like that after spending the early game feeding and doing no farming I have no idea but I did feel good about myself for overcoming such a huge disadvantage. We won if I recall. Teemo was still mad at me for trolling him back, his own fault for being miserable.
Othertimes the rest of the team and I troll abusive teammates by repeatedly declining their surrender vote spam even though we are certain we can't win. Once or twice that resulted in a victory if i recall.