I'm not familiar with RDesktop, but I imagine it is similar to the previous version of Windows Terminal Services. It would give you the desktop of the machine you wished to control, but not individual applications. VNC or AJAX would be fine, but wouldn't give you the illusion of running an application locally.
MS Terminal Services "Remote Application" gives the illusion you are running the software on your desktop i.e it only "transmits" the application window. So theoretically, you could "run" Office 2007 on a Win 98 desktop. All the processing is done by the TS Server, while the application appears to be running right in front of you. It works fantastically (I have it sending VMWare workstation instances to client P.Cs at work. It's just like it was installed locally)
I was musing that if a TS Client was developed for the Amiga Workbench, one could run very powerful applications - which were not actually run on the Amiga hardware itself. The whole process is, I agree, not practical, but it would be fun to use Workbench on a day to day basis again as opposed to "just for fun".
See here :
http://technet.microsoft.com/en-us/library/cc733093.aspx