My example of ornaments may have been a bad one. I wasn't sure how scientists saw it. But as a lay person, the explanations seem weak.
If you study bird behavior it is not weak at all. In many bird species sexual/courtship displays are extremely important in finding a mate. Some of the most extravagant can be seen in the birds of paradise. Color displays are used by hundreds of species to tell predators that they are poisonous. Some non-poisonous species even copy the color displays of poisonous species to reduce predation (e.g. coral snakes and king snakes).
Mr Jack, your reply, "some may quibble with the 'by natural selection' bit" tells me there are cases of evolution for which at least a percentage of scientists struggle to see how natural selection is responsible.
There are probably millions of such cases. That's why science is so much fun, there are millions of things to figure out. The important thing here is that the theory of evolution has been a very useful tool in figuring these things out. At the genetic level, the theory of evolution is an indispensible tool for finding genes that are under selection and might be responsible for specific characteristics. Scientists use the theory of evolution because it works. If some other theory is proposed and it works better then scientists will use that theory.