I'm not a climber at all, but I've done a little bit of climbing on an indoor wall, and a crowded day means you need to wait longer, which is a hassle, but I get the feeling there's a much clearer sense of how to wait for your turn, with less interference when it is your turn. No idea if this is the case for outdoor climbing, though, or at more advanced sites. I'd still prefer a less crowded day with shorter lines of course - but crowds on a surf break don't just make it take longer to get the experience of a ride, they can pretty well ruin it. That's a big part of why I only surf uncrowded beach break.
Crowds can be a problem at good beginner-friendly single-pitch crags, but yeah lining up for a popular route is pretty straightforward if annoying.