Two-stage tendering was invented for situations in which the client had to start building as a matter of urgency, yet retain an element of competition between contractors
As soon as the design reached a point where elements could be priced competitively, contractors were invited to do so. As they became firmed up, they would be re-priced. If too much of the project was left to be finalised before a contractor was appointed, there was the risk that he would not have been the lowest tenderer in a single-stage scheme. But the client would have to take that risk if he wanted his building in a hurry.
I believe that the bulk of the construction industry would prefer to leave the design to the designers. Two-stage tendering will always have its place, but we should not write off the traditional single-stage route.