Google is very good about finding sites this way. Inktomi used to be good at this, but with their paid program now in place, I'm not sure if they're still spidering like they used to. Yeah, they may spider and add you, but it takes long enough to be listed even when you submit. If you haven't actually optimized your site content and tags, then I would definitely say not to bother submitting. If you're on a very limited budget, then your Google listing may certainly be good enough.
Some sites can get tons, and others get only a couple of visitors a day.