The stock pistons did not have the holes because the bridge was relieved from the factory. The bridge gets hot and swells up from the exhaust heat and closes up the clearance on the piston. Usually when people bore engines they do not relieve the bridge, so they seize, same with a replated cylinder. The holes supply some lubrication to the bridge area, and more importantly they cool the bridge and keep it from swelling. With all that said, I do not drill my pistons, but I always make sure the bridge is relieved. As for Wiseco pistons running less clearance than stock, that is not true, Wiseco pistons are forged, and the factory pistons are cast, a cast piston does not swell as much as a forged piston when hot, so a cast piston does not require as much clearance. The old Wiseco pistons were very bad about seizing, the new stuff is top notch, still I have not been able to run an engine as tight on the clearance as what they say and not scuff the piston, at least not on the high horsepower air cooled stuff.
Got a 75 CR250 in here right now getting some very trick work done, it will be interesting, it should be faster than my 74 engines, which is kinda scary to think about! 