Should the 'tool' tag be removed? It's kind of broad and undefined. Besides, blender is a tool. I don't think using it to mark a question about the knife 'tool' makes sense either; that's too broad.
The tool tag has been removed from the questions they were assigned. Tags with no questions are soon removed completely. If you see them pop up again, flag one of the questions for moderator attention. This becomes less of a problem after the private beta, when the rep requirements to create new tags is increased: https://blender.stackexchange.com/faq#reputation.
A question about the knife tool is fine, but tool needs to go. It's not useful because it could refer to anything or nothing at all.