I have an affinity for speed and my speedo is always in shot, and because I'm a paranoid shit I don't want that footage to be traceable back to me. I don't blur the speedo but I blur my plate, and my face is never in view.
By affinity for speed I mean if it's a 30 and the road is clear I'll be above 30. That doesn't mean I do like 50+ but I'll be say 36 or 43 or something. I just ride at my own pace and typically that's above the limit a tad.
Not only that, but I'd rather keep my privacy in check because I don't want people (again, paranoia) to know where I live and as such where my bike is parked.
I think it's preference really.