YP sites are mostly geared towards B&M businesses (since they usually have a geo component for searches). Many will not even let you submit a business without an address (and some will delete it if you fake an address).
So if you are not a B&M business, it's probably not worth the effort. But if you are a B&M business, then submitting and confirming the correctness of your listing information across all the major YP sites is essential for SEO.
Like anything, spread it around (don't forget Bing, Yahoo, duckduckgo, Yandex or other SEs). YP most likely will not give the best bang for the buck, but if you have a dollar to spare spend it. Getting the site out there to the users is what is required. That brings in the eyeballs. What you do ON SITE for those eyeballs will determine your success or failure.
MANY paths to example.com is far better than courting only one... or two. A YP listing won't hurt you. It just may not help as much as you'd like.