I don't know! I am not very sure about the widespread cliam that pinging your blog's feed URL will drive free traffic. I tried it several times. Nothing happened. May be I should tried to monitor the traffic using special tracking methods.
Do you know the most talked about pining service called Pingomatic? It notifies plenty of search engines that the submitted blog is just updated with fresh content. Don't search eventually crawl blogs? What is the necessity in using pinging services?
One view is that search engines do send their bots to blogs periodically but it takes time, After all, they are too busy and the launching of new blogs make the process more delayed. That is why pinging your blog speed up the visit of search engines.
Is it so?
As a problogger, I have subscribed to pingomatic's blog feed. It was last updated on March 2009!