Suggesting one unblock ads is fine. Expecting it to make any difference isn't.
The web is designed fr *user* control, not the other way around. It's up to the user (and the browser) to decide which content to display, and how to display it. The page author can provide *suggestions* on how that content is to be displayed, but the browser is free to ignore it.
In some cases, the browser is simply incapable of showing the content - say, images in lynx running on a text-only terminal, or a browser designed to assist the blind. In others, it may be due to user intervention - blocking images to reduce bandwidth usage, for example, or applying a custom stylesheet to reformat content for a small screen.
Or, it may simply be a case of a user not wishing to expose themselves or their computer to ads. There's enough malicious code out there being propagated by ads that it's probably in the user's best interests to block them (particularly if you're running Windows). Most webmasters aren't serving their own ads - they use an ad network. These networks don't exactly have the best track record in preventing malicious code from making it out to users. They tend to only act reactively to such things - at which point the damage has already been done. This of course isn't the webmaster's fault, but it means a webmaster simply doesn't have the control over the ads necessary to be able to make the judgement that "the ads my site serves aren't annoying/intrusive/malicious". They just don't *know* that, because they aren't the ones serving the ads.
Not to mention the fact that if a tag like this becomes common, *all* webmasters will add it, regardless of content. This will make the "This author says their ads are good, view them?" statement meaningless.