Telling a player they should quit? Is that ever justified?

[Nihility]

not for poor skill it's not. Party finder seems to have tools to weed out those that lie about their ability fairly well lately though

there are some people that just go on and on and on and on about how much they hate the game but are always around. Might be warranted to ask them why they don't quit
there are the game addicted too that let their real life fall to shambles while fixating on a game. While not something you normally know about someone they should probably quit

[KokonoeAiyoko]

If you're telling someone to quit because they don't play as good as you want them to, no that is never justified and you would be an absolute jerk to say that.

[NessaWyvern]

The only time someone should be told to quit the game, is if the game is literally detrimental to their health.

When there was speed hackers in the first season of the feast, my husband told me to quit it for a while because I was getting too salty and upset.
It was good advice, because I felt so much better after quitting it.

Back in the day, when titan extreme came out, and they hadn't fixed the bad latency in there, I used to get hit with landslides (New Zealand ping didn't help). I got told a few times to uninstall the game because "It's not latency, you're just bad". (When the fix came, I was magically able to clear it easily, how odd.). I found it very upsetting at the time.
Don't listen to those kinds of people. Those people are just jerks, and this is a game you paid for, and if you enjoy it, keep playing it.
Also, if you see someone telling someone to quit the game, don't stay silent about it. Speak up for the victim, make them feel better.

[Kewitt]

I had to force my hubby to quit. While he was in school But now that he is working full time.. He can play any game he wants.