Hein, I agree with you. I have never understood the reason for this breeding, like phrag besseae x dell'asandroi (sp?) other than for "hybrid vigor" but then again, at one point they were considered the same species so would it have been called sib vigor and would it even have existed (the vigor part)??? And another thing, how would you know that a hybrid made a while ago when they were considered the same species was not recreated with say a jackii and labled as the same hybrid? Ah yes...taxonomy! Don't you think it should be all DNA based?
I am a breeder, so I can answer why you would cross 2 varieties of a species (like Laelia purpurata schusteriana and Laelia purpurata carnea) or 2 species that look very similar (like Phrag. besseae and Phrag. dalessandroi).
Like Leo said earlier, one of the main reasons is: vigor, vigor, and more vigor. A lot of populations have been isolated from each other, and some populations have been pretty small, so over time you get a lot of inbreeding, resulting in weaker plants. Also when people started propagating those plants (say a laelia purpurata carnea) they would usually only have one plant, and thus they would self it. The resulting offspring they would sib 2 plants to get more offspring. As a result you would have a lot of inbreeding going on. Now if you cross 2 plants from 2 different populations (like the carnea form with the schusteriana form)....voila.....you get Hybrid vigor. (kind of how they create Hybrid corn). Another result of crossing the 2 populations, it that you can create some interesting color combinations. And that is what you want as a breeder: more variation.
Another reason for crossing 2 related species (like Phrag. besseae and Phrag. dalessandroi), besides getting more vigor, is to combine characteristics of both parents. Phrag. besseae tends to have better shaped flowers compared to Phrag. dalessandroi, but the nice thing about dalessandroi is that it has branching inflorescences with lots of flowers that are open at once. Now if you combine the 2, the breeder will be selecting for plants that have the flower shape of the besseae plant, and the number of flowers of the dalessandroi plant.
I hope this answered some of your questions,