They are worth it in some cases. As stated, you can get bigger, nicer looking corals for less $, but will they hold their colors? All of my tyree corals look as advertised or very close. The money spent somewhat guarantees they will look a certain way, which is almost impossible to claim with most others. So if you see a certain tyree coral you like, most likely it will look similar in your tank unless you have drastically different conditions. Many of his corals are also proven to be quite hardy. The purple monster for example, many times I've heard and seen it the sole survivor of major tank crashes. I have corals from other "high profile" type growers and very few of them maintain colors as advertised.
This also doesnt take into consideration the aquaculuring argument already made, which is very valuable IMO.