I've been wondering why this is because I really do think their appearance is very much improved. The conclusion I've come to is that the shine is so much more natural (forget all the other stuff like bump mapped rivets etc - they'd never show up with my card).
When you increase the alpha in FS9 to increase the shine the underlying colours wash out the higher you go. But that doesn't seem to be happening in FSX. From what I see I'm pretty sure that's true.
Any other keen painters out there come to the same conclusion?