I think that many of the reasons for using citrix might be removed with Longhorn, not for sure, but there were rumblings of some of the features being implemented. At least for smaller organizations. Although I've seen it implemented at smaller places and the most I could get for the reason for it was so that people could connect to a remote desktop on the server.
A few years ago I needed to do give a remote users at smaller satellite offices access to the accounting program. Due to the size of the company I couldn't justify citrix, so I just had them all connected through vpn and published the accounting program with a program from here http://www.2x.com/
Worked really well, and for a lot cheaper.