> I understand that all designers are impractical people who will happily produce something insane and impossible and then insist that it's brilliant in the face of all objection.
Any person who does that is not a designer at all. Designers make extensive previous user-centered research, with tools like etnographic research, questionnaires and others. And designers, during the development of their products, do EXTENSIVE and INTENSIVE prototyping and testing WITH THEIR TARGET AUDIENCES, using A/B testing and other user testing tools. Engineers and software developers, OTOH, tend to think mistakingly they are the target audience, and they have vices like engraving the product or service with their own preferences, or removing important (from the point of view of the user) features, or, worse yet, setting novelty features as default (which breaks the user workflow 99% of the time).
Good design drives development in the direction of the user. The developers should help with the input and point the possibility of technical problems, or technically solve those problems. It's just like air traffic controllers show pilots where they should take their planes. Pilots still have to control the plane, but the important thing is that THE PASSENGERS arrive where they want to.