I raised the same question at Brady v Dalen shop, who refinished my WTB and Zieleman. Brady is the brother from master builder Jan v Dalen, the person behind Duell. Bradys answer was simply that overhere (Holland) you can't sell conventional lugged bikes to the audiance Jan builds for. Jan builds serious performance aimed steel roadbikes.

www.duell.nl

Jan started with lugged decades ago, but now it is all welding and unconventional shaped tubing.

As some other posters stated before: Welding is not only here for reason of being cheap. I happen to own two Grove Innovations mountainbikes. Both bikes show supersmooth almost invisable welds. Doing it this way you will absolutely not achieve an efficiency gain over lugged. A lugged frame could be put together with much more relaxed tolerances.