Traffic does not come from a single source and therefore just targeting one avenue would not yield the desired results. It sure can help you augment your existing traffic. Try dmoz and the likes as they are know to deliver high traffic, if that is what you are looking for.
I dont think so one site can wonder and bye the way it is paid. second if you want to drag yahoo bot attention just add your website in site explorer.
If you want traffic: NO. If you want links, page rank: NO They have become an "also ran" with just a lot of fluff.
If you have the money being in the Yahoo directory is a smart move. Not for the traffic but purely for the link. And it's not a link you can control the anchor text with, it's purely an authoritative link. Google loves sites in the Yahoo directory because they know the website had to go through human evaluation before it was included, so it must be good. Think about it it makes sense, google only wants quality sites in its index so what better way to do that than boost sites that have been reviewed by a human. It will obviously help your rankings in Yahoo also.