Isn't NFS using "any port number and advertising it" exactly the same as any other program asking for "any port", and that port happens to be a reserved port?
I have this problem on my windows PC at work: Windows assigns misc ports in a range that JBoss expects to be able to bind to, so if I don't start JBoss first it may not start at all. I work around it by shifting JBoss's ports to some other ports which are not allocated right after boot. But this doesn't really solve the problem.
This feature lets me say to the OS that it should never hand out a specific port unless an app requests it. Yes, you have a race if two processes want to open port 80* as a web server, but now you at least don't have to worry about _something else_ opening port 80 by accident and blocking your webserver.
Sounds like a good idea to me.
*(ok, that port isn't usally one of the randomly assigned ones, but it serves an a example)