the way i see it is electricity always finds the shortest possible route, once your coils have a build up on the inside, the usual route is no longer the best one. so you all use araldite, but you are then no longer giving it a nother way to go except down to the coil, which is fine, but its always going to be "searching" for an alternate route.
SO by cleaning the contact on the inside of the coil you are making the proper route down to the spark plug the best possible one.
im no mechanic, but ive had my fair share of electronic experience (breaking stuff) and after i sanded down the inside of the coil packs, my cars performance increased noticeably