Having a filter applied to an entire track, or just a clip, is rather handy when you need to add some universal effect to it. However, it is rather limiting in a few ways. For example, maybe you want an effect to be animated, fading in and out.
I think that having a filter as track could be helpful in many ways. For example, you could apply it to multiple video tracks underneath, or being able to animate the effect, like fading a filter in. This would be very helpful when you need to apply the filters to multiple clips, but not all clips.