The advantage of the Budd Patent all electric welding monocoque construction which meant a lighter weight vehicle able to take buffing forces compared to traditional construction. ( Underframe trussing was not structurally required, the LMS bought some all steel carriages from Leeds Forge which also lacked trussing, ref "LMS Coaches" )
Lighter weight meant more carriages, and therefore seats for a given weight.
Also being all stainless steel there was not the problem of rusted sills due to moisture accumulation. Hence Chris Leigh's comment about long life in North America ( and Australia ).
When Amtrak decided to rebuild inherited carriages it was the Budd Patent carriages that survived due to a lack of corrosion compared to other carriage builders who attached stainless steel covering to non stainless underpinnings with the result that with age internal corrosion became a major maintenance issue.
From BR's perspective the Budd patent carriages would all have to be bought from the one UK licensee, Pressed Steel, as the process was unavailable to other builders.
