I won't say that we don't have intelligent people making the decisions on what to replace and where to cut, but sometimes I have to wonder....
Ask him about sockets and butts. Yes, we do a lot of "pre-fab" (built in shop or an off-hull work area) work, but there is often a lot of pipe welding done on the boats.
Present practice? We're repairing / upgrading boats built 30 years ago. We don't do new construction, and aside from the Seawolf and Virginia classes, there aren't very many "new" boats pulling into our drydocks.
I don't know which shipyard your inspector is from, but at the one I work at typically only P1 (high pressure/temperature) or nuc systems require mock-up training.
Replacing the defective pipe with a flanged pipe means you still have to weld flanges onto the cut ends of existing pipe.
There are always those "last minute" repairs, too. Test failures, installation issues, someone working another item dings/gouges a pipe beyond minimum wall thickness... the list goes on.
Perhaps your inspector is working at a yard (east coast?) that is not comfortable with allowing the weldors to properly develop and utilize their skills.
Mirror welding is not a royal pain in the butt. It is a skill that is developed by practicing just like any other. There are lots of very proficient mirror weldors out there, and I have seen several complete joints that I wouldn't ever want to touch. I enjoy the challenge of mirror welding, and look forward to opportunities to use the skill, since it means I don't have to break my back trying to bend around obstactles to see the entirety of the joint. People who avoid it and refuse to practice can only expect bad results, and often don't like it when a proficient mirror weldor completes difficult joints. It makes those who refuse to try look bad. It's one thing to know your current limitations, it's another to refuse to attempt to learn a new skill.