I am no expert but as far as I know you dont need to submit to the Yahoo directory to be in Yahoo search listings. If I am wrong I am sure someone will say so.
Not necessary. Yahoo directory submission is a paid one. You have to pay $299 per year to yahoo for that.
The Directory and the search engine are not the same. most sites in Yahoo! search are indexed for free. the non-refundable $299 fee is for maintain the listing in the Yahoo! Directory. they do have free submission but getting listed could take alot of time.
You dont need to submit your directories to yahoo to be part of yahoo results. If your content is up to the perfections, it will be searched by yahoo ultimately.
certainly it is important, as still many people uses yahoo to get search result.. You can submit your URL to yahoo here
It used to be worth it back in the day. I paid it once and never paid it again. My link is still there today.
As everyone already said, you don't have to submit your site to their directory to get indexed in their search engine.