> But RDP gets a lot of other things right,
> which Unix/X11 forwarding doesn't
You forgot performance. I've tried using X11 forwarding over SSH on a 100Mbit LAN. Maybe I'm doing somethng wrong, but I always found the performance to be abysmal. They say it's GTK's fault, but what's the point of a feature if major toolkits can't find developers to work on properly supporting it? As it stands, I couldn't care less about X11's network transparency.
RDP, on the other hand, flies even over a regular DSL connection. That's more important than getting color profiles right.