Though you have a point (responding to incognitus's post), the employers will like it but thats it. Everyone else who watches the new showreel will just see what's mostly the old showreel and not care for it.
So you just have to not mind the score i suppose, as long as the employers see it. I'd say wait a long while before releasing a new one and provide more of your latest work, with less, (but still some) of your past work too.
Incognitus
"Man, Newgrounds seems to be treating the showreel harshly. Why exactly, I dunno."
It's mostly footage from the last few reels you've submitted here. When you have a "2010" demo reel it should be comprised of work from that year solely. It looks better to have yearly reels that actually focus on said year to show improvement and the growing range of projects you've tackled. Simple as that. When you have cut-and-paste footage of the same stuff we've seen it's not going to be highly praised.
Wonchop
A lot of people here don't seem to get the idea of showreels. The idea is to present your overall best pieces of animation so that potential employers may like it. The date is more a reference of how up to date the showreel is as opposed to what year the entire content was made.