I had considered that, but ultimately settled on the single lens, as it is easier to see the difference in a large on/off light than the more subtle variance in a stoplight. Also, most forums use some type of on/off metaphor for their index page (heck, look at the default theme), so it's clearer than the stoplights, which might make someone go "Stop? Go? Oh, the green lights have new posts."
I remember one of the old themes during the Cody administration used interstate shields for new posts and US shields for no new posts. I remember seeing that design choice being lambasted on MTR for being "too complex". And this is from a roadgeek...