Blitz/Thresh promote toxicity.
When you build a champion that encourages players to figure out who has the worst reflexes on the enemy team and grab them all day for free kills, you're just making the game a worse place. This is much worse with Blitz than Thresh in this specific area, but they can both cause the same problems. One, the normal tactics of the game start to get ignored, because it becomes all about baiting that one guy to get grabbed. Pushing objectives becomes nearly impossible, because a guy on your team can't dodge a grab to save his life, and every time you push he gets yanked under tower and erased, then it's a 4v5. People on your team are mad that blitz/thresh is trolling them so hard, and they often get mad at the poor guy who has been singled out. This causes all kinds of problems. People won't push at all anymore because 'dude will just feed', so we can't take an objective, but people also won't surrender because they'd rather sit there and do nothing for 10 more minutes just so they can yell at the guy whose game is getting ruined by mr. grab. Is all this really worth a random cool video here and there and the smiles of people who like to troll others while technically following the rules?
I mean Blitz specifically is horrible in this aspect. He doesn't even translate into much for the team lategame unless there is someone he can abuse with grabs, so it's like his entire purpose is to be a troll. He's still often effective because his trolling can get an adc more fed than a Master Yi in bronze(and his existence in a lane basically destroys your opponents farm instantly), but are these really appropriate/healthy ways to be effective?
Edit: Yay forums. Not much disagreeing, but people sure do like to downvote. I'm sorry if you like trolling people but it has no place in the game :P