The only reasons website designers historically have not followed web standards are (1) not being aware of the standards at all, or (2) to make non-standard-compliant browsers (*cough IE cough) do cool stuff. But Internet Explorer has recently been making a significant move towards standards compatibility, so reason (2) is becoming less and less relevant, and you're obviously not subject to reason (1), so why not write standard-compliant websites? As others have said, it will make your code much easier to debug because you only have to work with one standard rather than a bunch of different browsers.
