To your point, that's why enterprises pay for support... but it is really expensive. Hence why rh is doing so well.
So, it's not something inherent to Linux but simply an issue of adoption. If more people used the desktop it would be more solid. That would mean less bugs reported and it would be cheaper to provide support to everyone. I've no idea at what level of adoption this would occur, though, but, IMHO, the Linux desktops (that is, the complete environments not just people running wms) are pretty far from the stability of windows/Mac.