Quite true. The appearance of rational design does not help us in distinguishing design in a clear-cut manner. But it aids in assessing the degree of our suspicion of design. It is a clue.
But if it doesn't help in distinguishing design, then it
isn't a clue.
Well, I guess it depends on what your goals are... If you just want to feel better about your suspicions, and don't care about logic, then I guess what you say is true.
But then, too, the native americans had a lot of clues that their rain dances worked, right?
For example, if the structure of the flagellum was poorly designed and hodge-podge, wouldn't that be a popular argument against the idea that the flagellum was designed? But it is not hodge-podge, so it is one clue in favor of viewing it as designed.
Designed by the process outlined in the Theory of Evolution, sure. Nobody thinks it was randomly assembled. But we still don't have anything about it being purposefully designed.
Poor design is evidence against the thesis of a rational designer, is it not?
I don't think so. Rational designers can make poor designs, even on purpose.