In reply to her_t6:
As others have said, knees can be great. There's a 2 classic boulder problems in the Peak that I do by putting my knee on the lip of a roof so I can make the hand-move - we're not on about knee-bars here, we talking about using the knee as a foot. It works and I'm sure it's the most efficient method.
However, crawling onto a ledge on your knees *is* genuinely climbing badly. It's a bad way to approach getting onto the ledge, it's not efficient, it can leave you in a ridiculous can't-move position, and of course it's really ugly. This latter point is of course just a spurious value judgement, but surely we do all want to climb nicely as that's a huge part of the fun. I hate scraping my way up stuff, looking like a tit who's well out of their depth. I like the incredibly rare occasions when I climb smoothly, confidently and efficiently. Crawling when you should be climbing is the antithesis of this.