You or I or anyone else can make those judgments at any time about any number of people.
However… if you make the judgment based on no knowledge of the person himself, but just his membership in some other group or class that you use to categorize people, then it’s prejudice. You can still do it – many people do, obviously! – but many would judge you “useless” if you used that criterion.
And if you make the judgment based on only one action, appearance or behavior, excluding all else about the person except, for instance, “She cut me off in traffic! She’s useless!”, then that’s a pretty superficial and uninformed judgment. The same could easily be applied to you.
And if you make the judgment based solely on the person’s particular lack of utility to you, well, that makes you a pretty complete narcissist, doesn’t it?
The more I think about it, the less reason – and support – I can muster to enable a supportable claim of “uselessness” against much of anyone.
In any case, you’re free to make the judgment. You’re just not often free to indulge in any precipitate action against the person based on your judgment. In other words, you can’t kill ‘em.