In providing a (usually) free service, there is some understanding that you will provide certain genuine information about yourself in exchange for that service, in the form a registration/signup. By using a generic account, you are circumventing this exchange.

Sure, there's a good argument for anonimity, but it's give-and-take in the virtual world. You're basically getting something for nothing and robbing the site owner of their ability to proscribe the means by which new users are given access to the site.