Worthless: Having no real value or use.
If you think that a question or an answer has no real value or use, then saying so is entirely fair comment. It's possibly a little blunt and I guess you could couch your opinion in weasel-words ("I think your question is lacking in real value or use") to lessen the impact on those of a fragile disposition but the meaning would be identical and should not go unsaid.
While certain terms have been outlawed by generally common agreement ("rep-whoring" for example) I think we're all grown up enough here to cope with some honest feedback from our fellow users, even if there's the possibility that that person might not like what's they're being told.
- Don't call someone worthless. That's not nice under any circumstance
- If you think their question is worthless (and want to say so) then use the word advisedly, sparingly and carefully. It can have quite a big impact and there's a possibility they may not like it, even if it's a fair comment in your opinion.
- The above doesn't mean we should outlaw the word. If you think it's being used to attack a user, flag it and let a moderator review any usage on a case-by-case basis.