BEWARE the company that says they will submit a site to over 3500 search engines. First of all, there are about 12-14 search engines that a web site owner should be concerned with (the rest are just Link Popularity brownie points). Secondly, they are doing only about 5% of what is needed to rank well.
In fact, it is not mandatory to submit a site to Google anymore. The best method is to have Google find the page through a link from another web site with good link popularity. Once the site has been indexed, Google will refresh their database every 30 days and the changes made will be reflected. If the pages are already listed with Google, it is a safe bet your web site will be penalized if the same pages are submitted to their queue over and over.
Unfortunately, most search engines Add URL pages have been spammed and literally bombarded with unsavory web sites in the last few years, that search engines have had to put less and less importance on the submissions. A web site will rank much better if it has good links than if it is submitted to a hundred search engines.
That said, submitting new pages in an existing web site and pages in a brand new web site should still be done. There will be a time when submissions are a thing of the past, but currently they still hold a little weight, and should therefore be performed.