I had a bad experience once with a welded diff, one of the gears broke and a tooth got wedged between the ring & pinion. The rear end locked up at hwy speed and I slid to a stop. All I could do was steer it so it went off the road at the right time as not to get in an accident. Ever since then I have always covered the welded gears so any chunks can't get into the R&P.
Also have you ever seen a carrier break in two from the twisting load? I have, big tires and a welded diff can cause metal fatigue with the big openings and they break, not to mention just any flex in the carrier will allow the pattern to change causing a failure of the gears.
