Most applications will only be doing this username -> ID lookup once per user when they start tracking that user. The overhead there is minimal and drives applications to track users by ID instead of username, making sure the same person coming back will be treated as the same user, even after a name change.
But, it’s a significant overhead for simple applications that do not track users over time (every API request turns into two), which I think is what @mkdr is talking about. I agree that it would be nice to have a way to request directly by name on all endpoints where applicable for this reason, however, it would need to be in a way that still encourages developers to track by ID. I don’t think now, during the transition period from v3’s username tracking to v5’s ID-based system, is a good timing for adding such feature though.