Plenty of Unix systems could be booted from NFS. Assuming no local storage, and that you need X, the question is, what's the difference in cost and capability between a X terminal and a diskless unix station that runs X? You've also got to consider costs to run a NFS server for your diskless unix stations vs costs to run a desktop server that backs all the X sessions.
If you think about a lab full of computers doing relatively simple Unix work, and how much money would be saved by just having a single drive (and all other things being equal, which they of course aren't), it's not trivial.