I asked Bernie this very question when he plugged and welded my fork. He looked at me like I was crazy and said that they all eventually strip. I didn't get it right away, but, if you look at the thickness of the tubing on the steerer tube on a threadless fork and then compare that to a threaded one...

If you're gonna get your fork welded, get someone good. There are not many people I would trust to do this modification...