They figure that they can take your money, do the bare minimum of submitting your URLs to the search engines - and just walk away. Well, guess what. So will your potential customers. It's one thing for your business' URLs to appear once or twice in the search engines. Their turnover rate is remarkable.
Listings change daily.
Placement fluctuates like a seasick jellyfish. Once is not enough. The changes in engine policies. The typos the other guys stuck you with. We fire all of our guns at once - so that you explode into webspace.