Going back to the original question (in the hopes of preventing a war...)
After chatting to a mate, the only problem we could come up with regarding ERW is the chance of fatigue.
Due to the high levels of vibration the bones will experience, the greatest chance of causing problems during normal driving (excluding bricks, kerbs, other cars, etc...) will be from fatigue.
The weld in the ERW will act as a harder, more brittle area and as such will be
slightly more susceptible to fatigue. Presumably this is more than made up for by the slightly larger tube thickness.
Thoughts? (The seeds of a new war have been sown, the facilitator sits back to watch the chaos...
)
And yes, this discussion was held down the pub!