when its accurate. like when you have a full health midlaner that won't help the jungler fight his lane opponent in the river brush right below his lane. you can call the person useless.
also, being called useless is probably one of the nicest versions of toxic, and its probably coming from a player that generally has a low toxicity problem.
I mean, if you want to imagine real toxic imagine a player intentionally feeding the whole game because he didn't like the fact that you picked taric in a ranked game. or imagine a lee sin deciding that even though you had first pick for jungle and had picked jungle, he was going to pick jungle and proceed to follow you around last hitting everything that you try to fight. or someone calling you things worse than useless throughout an entire game with no reasoning or logic behind it.