Okay, I do it occasionally but I try hard not to do it loudly. Here's why:
For obvious reasons, I take my rear foot out of it's binding to ride a chairlift. If any snow get's into that binding, I try to get it out ASAP.
If I don't, I will end up riding with a layer of snow between my boot and my board (Not comfortable or fun). The pressure that my foot puts onto that snow then turns it into ice which is even more uncomfortable, and MUCH harder to remove.
The easiest way to get the snow out of the binding is by slapping the board on the ground. (Sorry, I can't always be bothered to bend over and wipe it out with my hands, getting my gloves wet in the process).
Wiping it out with the rear foot usually doesn't work and ends up compacting snow onto the flat of the binding (exactly what I'm trying to avoid). It also increases the risk of me falling flat on my face (and on to your skis, which would make you even more mad).
To conclude: There is a good reason for it, but there is no need to do it loudly. People who do it badly are douchenozzles.
