Many Brompton resellers propose the Bordo to their customers because it can easily be transported on the Brompton.

There are plenty of reports of stolen Brompton "protected" by a Bordo!

I agree with you that cutting a chin with a portable grinder is more difficult than cutting a U-lock but a strong chain (it must be thick enough, otherwise it can be cut by a bold cutter) is heavy and difficult to carry on a bike while its easy to carry a U-lock with a dedicated mounting system.

There are now portable grinder resistant U locks like the Hiplok D1000 and Litelok X3.