alexa is a tools, its help when you buy or sell a site. you can guess how much traffic the site getting with alexa rank, even though its easy to cheat
that why i prefer Google Analytic to check my own traffic, alexa is only help me to measure other people website details. Then we only can take alexa as a referral. But i still love to use alexa to search for top website in the country which help me lot in marketing plan.
I agree with you. however still lot of advetisers check alexa ranks before they provide advertisements, and some people also really care about alexa ranks.
The top 400,000 is fairly accurate. If you check both YBL and Alexa, it should give you a decent idea about the site's popularity, so don't be so b**chy about Alexa, it provides some valuable insights