Programmers have their tales, much like sailors. One in particular concerns the ravening schools of email address scrapers that swim through the web, tearing each juicy morsel from the safety of its page, then conveying it down to the spam masters who dwell in the frigid, dead, depths. So it is that no wise person puts a plain email address out on the web - unless of course they like being buried in spam. The oldest of my personal addresses, that have been seated upon webpages continuously for going on a decade now, are floating around the 99% spam mark. No joke, that.
One widely used method of defending against scrapers is not to use mailto links at all. Instead print an obfuscated address in some format that a human can use to derive the real address - but which a scraper will not understand. "fake.address -at- exratione -dot- com" for example. This seems like giving up, however.
To defend against scrapers while keeping a functional mailto link, you'd have to think a little about the capabilities of the scraper you want to fool.
Naive HTML scraper
A naive scraper grabs the page HTML and parses out email addresses. No processing, very simple, very fast. I don't doubt there are many, many of these things roaming around out there. It's easy to ensure that a naive scraper doesn't find your email address - just make sure it doesn't appear in the HTML. For example:
DOM scraper with event processing
Even a simple event processing approach to an email address scraper would circumvent the jQquery snippet above, needless to say.
Now consider that amongst the people who have your email address in their address book, half a dozen have probably already cheerfully uploaded your address to a one or more of grasping online services, or fall victim to some other address book pillaging scam. Most of the people you send email to will have no incentive to keep your address private, and will hand it over to third parties without any conditions placed upon its use. From that starting point there are a hundred ways for an email address to make its way to the black hats and spam houses.
That said, if five minutes of thought and a few lines of code means that the least sophisticated scrapers can't add my mail to their stockpiles, at least I'll have the comfort of knowing that it was probably some other fool's fault once the spam starts to trickle in.