I am also interested in this. From what I can tell, the only place this comes in to play (generally) is for Hipersockets and similar private networks.
In general, in a network where there are significant differences in MTU in different parts of the network, and where/when latency is a significant issue. When I asked my network people about it, they said that the function has to be enabled and supported through all the hops in the network (many of which are external network providers in our case), AND that firewalls had to allow the ICMP traffic through. Both correct. There is also a rumor floating around that PATHMTU results in the "do not fragment' bit always being set on. Which seems very scary and wrong. That was true of a buggy release of the Windows TCP stack a long while ago. That's been fixed mostly, but since most people use Ethernet inside their network these days, and things that use Ethernet framing outside local segments, the value of path MTU discovery is less useful than it used to be. I just turn it off and let the routers deal with it - they have plenty of CPU, and the MTU changes so rarely in most networks that it's not worth it.
