They do, but 10 minutes with a small drill bit sorts that out if youre worried by it. Takes quite a while for them to be completely blocked so not something you need worry about every day.
To my mind the fact that they get blocked kind of gives the game away that they might not be doing much good. They're supposed to be there to release gasses or water trapped between the pad and disc. If they did that wouldnt it keep the holes clear? Personally I prefer slotted, if for no other reason than it appears to stop the pads glazing, or just plain solid if the car appears under-braked.