One compelling reason to not date your articles is that age sometimes destroys perceived value even if it doesn't destroy actual value. For example, I have a bunch of stuff writing about customer service written in 2006/2007. That field has not evolved so much that "Don't curse at your customers" is bad advice in 2009. However, if one of those were posted here, the first comment would likely be "Dude, mark in the tit…
Shouldn't the judge of evergreen-ness be from the reader, and not the author? If it really is timeless, people will come back and link to it again and again. I see the Norvig posts on learning to program in 10 years come up again and again here. It sounds like if you wrote books instead of blog posts, you'd rip the covers off so readers can't judge it by its cover.
Here's the fundamental difference between me-as-employee and me-as-businessman: I will get paid for my labor of January 19th on February 30th, and will never see another penny of it even if what I do today has lasting value. What I do after I get home, though, I get paid for every year if it has lasting value. I will not cheat myself out of that lasting value by setting an auto-expires header on it.