Although it's not worth much, it would take an aweful lot of work to get your site in the top 1,000 and I like the idea of rating websites, so I won't vote them worseless
Actually it is not easy to manipulate its ranking nowadays. They have made some changes and there is a long thread at DigitalPoint on this topic where the old methods to inflate rankings on Alexa no longer work was discussed.
As far as how relevant Alexa is, places like TextLinkAds and Adbrite for example use Alexa's ranking when sites are listed for advertisers to look at. In that respect, a nice ranking would help.
People seem to pay more for links on your site based on Alexa ranking (along with other things) so I guess it's still worth something. I personally don't use it but a lot of people still do.
It depends if you would be needing the alexa number to impress advertisers or not. Alexa has fixed their act and its no longer easy to inflate your alexa rank. Anyways alexa traffic numbers are not taken as a benchmark so if you need it use it or dont bother about it.