I've worked on projects where if you wait for the requirements to be firmed up, you'll never be able to do anything. Depends on what you're trying to build if that means you need to stop and figure out the requirements or if you need to just deal with the shifting sands. Aircraft built for moving requirements don't work so well; but lots of things are fine with moving requirements. It'd sure be nice to know how users are going to use your product before you build it, but sometimes you build what you think is wanted, and people only use part of it or use it for different things than what was intended, and it's better to adjust and refocus than to start a whole new development process with the found requirements.

Of course you should not wait around. I think rather the opposite that the engineers should be more involved in working on the requirements. The issue is more rushing and being expected to magically just conjure something. Changing requirements is a fact you just have to live with in many industries.