Humans get urls wrong too. And as g1smd says, Google will run forms and request urls to see what happens. Google will only index a certain amount of content from a site and seems happy to fill up your quota with non-existent pages with dupe/no content at the expense of proper content.
I've always fixed this as part of a site overhaul/optimisation so I can't say 100% that in itself it affects rankings (i.e. cleaned up non-existent pages in isolation and watched rankings improve), but my gut feeling is that it doesn't help the site's overall profile and if you don't have enough IBLs/PR to get all your pages indexed then you are definitely missing out on traffic.