I have configured everything needed to boot using PXE using NFS as
root-filesystem at home some months ago:
http://dpa.li/pxeboot.mp4

I export the root filesystem of an lxc container read only using NFS.
It's really convenient, I can install and remove anything I want in my
lxc container, and all Systems I booted using PXE will have the new
programs immediately. At the same time, the PCs can't make any changes
on the root file systems. But I don't actually use it, I just created
that system because I can.

I guess it would be pretty useful for large companies, if it weren't
so slow and insecure. I don't think I could use kerberos in that case,
but even if I could, I wouldn't want to use it. It's just so
overcomplicated, if I could just use it over TLS or SSH directly,
without any tricks, it would be so much easier. I don't even need
encryption in my case, a simple way to check if the datas came from
the correct place and weren't altered would be sufficient, but the
only thing there is to secure NFS is kerberos, the same thing used to
secure all MS stuff, the thing best used together with Active
Directory, I don't like that. I think what NFS really needs now are
simpler alternatives to kerberos.

But I don't think mounting just /usr using NFS is a good idea, not
because of NFS, but because it's technically a removable media, it may
not always be there, even worse, in this case, it may be used and
changed by other machines. I think the main problem here is that the
current package managers can't handle installing some parts of a
software on a removable media. It would be cool to be able to just
install some software on some usb sticks or something, and to add and
remove them when the software is needed, without the package manager
and possibly the rest of the system freaking out.