Richy,
I don't even know where to start on a thread like this....Personally, I disagree with virtually everything you talk about here. I've been dealing in stainless steel welding, technology, and engineering for nearly 35 years and have never heard anything about welding SS with a carbon steel filler metal because it allows more flexability at the joints. I'm not even sure what you're talking about. Then, you talk about the carbon steel preventing the "brittle" 316 joints from fracturing. Actually, a 316 SS has good ductility and elongation factors. Are you really suggesting that motor cross builders actualy braze SS tubes with carbon steel? Then, your next sentence about "cheaper grade s/s that isn't prone to cracking...etc., etc." totally makes no sense to me. Then, "....the corrosion at the joints isn't badly affected by using carbon steel due to the thickness of the mig weld bead would also take years to corrode" is a totally false statement. What in the world does the thickness of the mig weld bead using carbon steel to weld 316 SS have to do with the corrosion factor? Even if you were successful at welding a 316 SS with a carbon steel filler, it would be the weakest part of the weldment, and have the least amount of corrosion protection. You asked "is it bull?" Well, I think that is putting it mildly. When I read both of your threads, I asked myself, "Is this candid camera?" That is how ridiculous it sounds to me. Not trying to sound disrespectful, but someone has been messing with you.
Chuck