Well, it's worth understanding that for sites with no obvious need for user accounts (say, washingtonpost.com) the goal is to learn more about the audience so that they can provide aggregate demographics to advertisers and (if they're smart) target the ads to individual users.

So by messing with the accounts, you deprive them of that data. That said, I would assume they're smart enough to see that an account that gets logins from 100s of IPs a day is obviously not one person.

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.