I agree with the sentiment, and many talks do this really badly ("Here is our outline, we start with an introduction, and end with a summary"), but it is worth mentioning that the alternative isn't no structure at all, but trying to convey a bigger picture to your audience for them to anchor each section in once you actually start your talk. This could be done like the OP suggests ("Just tell people the key idea upfront"), but there are other ways: instead of telling people the end result, tell them the question you set out to answer, and present your talk as this journey. look at the same thing/topic through different lenses/perspectives. Present a rough outline of a proof you are going to go through, or a case study you are about to present before going through the details sequentially.
> How does one become a good collaborator? The golden rule: Do not block.
Not only is this great advice for effective collaboration, it is also a very nice habit to have in any place where people's impression of your ability determines your future (career) trajectory