Submitting to directories is not much important now except to some very very reputable and high PR like Dmoz, Jayde etc. Search engines is not counting the backlinks from directories now so if you submit your site to directory it won't give any good or bad result to your website. What directories can do is just they index your new website little fast so you can use them just to index your website in search engines. Search engines only rely on your website structure, content, internal link structure and all.
Directory submission will create more back links which helps in increasing ur page rank and SERP nowadays many people are using social bookmarking to increase PR & SERP
Don't listen to the idiocy above form chris312380. DMOZ is NOT crucial for anything. Anywho, to answer to OP's question; imho, they are not necessary, however, they are a great asset to help drive your keyword search high due to the anchor text associated with you listing.
I do not agree with you at all in that only few web directories like dmoz and jayde are important, there are thousands of quality web directories which give your website quality back links. We all know that there are quality web directories and useless spam web directories and submitting to the first will help your website gain more quality back links and thus, rank better in search engine, while submitting to the later is indeed waste of time and effort. Beside directory submission is a part of SEO particularly link building and its the most effective free method to gain back links to a website.