Most of the series is filler, when you think about it. In fact, the vast majority of the series is filler. We've gone 7 seasons with very little to indicate who the mother is, the Marshall and Lily storyline has moved at a snail's pace and so on.
At this point, the reason to watch the show isn't necessarily to expect the show to go anywhere, or for Ted to stop being such an emo dweeb, but the characters are so well defined and play off of each other so well, that its worth checking in just to see what they're up to. This week was a perfect example: one storyline about how we're really going nowhere with the whole mother angle, and another about how Barney likes to mess with Lily and Marshall. I'll be honest, I wouldn't enjoy this show so much if it was really all about the former and had none of the latter.