For the basic listing service, it certainly is a no-brainer. After all, it costs nothing and includes a free link to your Web site. I'd say that any SEO or marketer worth their salt needs to take the time to make sure that any client they have that qualifies for a listing is listed.
As for the upgrade to the enhanced listing, for $10 a month, it's certainly worth a short test run. Depending on the market, it's hard to say if anyone would actually find and click on the links (which may be why Yahoo! isn't imposing a per-click fee along with the monthly fee...they know that the traffic just isn't there yet.). Small markets (say Toledo, Ohio or Amarillo, Texas) may not have the traffic yet to make it worth the investment, but if you are in an area with a high rate of internet adoption (say San Fransisco, NYC, Chicago, Austin, etc...) it's likely a great investment.
Besides, when you compare $10 a month with the cost of your average Yellow Page ad, well...there's just no question of whether or not it's worth a try!