Why would you use half duplex on a network switch anymore?

I remember back in the military we would run our network at half duplex on the encrypted side, this was so that we could avoid collisions.

If you are plugged directly into a switch there will be no collisions. That is why you can run full duplex.

Plugged into a hub (or connecting to an older 10base2 or 10base5 segment) all the servers share a collision domain meaning that they have to check every time they send a packet to see if there is a collision in which case they wait an increasing but random amount of time then resend. On a busy network the same packet may need to be resent multiple times and if all hosts waited the same amount of time the resent packets could keep colliding with each other. The amount of time for resends increases with each failure so hosts will back off and overloaded networks can recover.

When servers are connected to a switch (or a crossover cable) no collisions are possible so both sides can transmit full speed without checking for collisions.

Back in the day I used to have entire class c networks all on the same 10base2 segment and I noticed some companies supernetting (opposite of subnetting) multiple class c networks putting several hundred workstations on the same segment (and it was slow as heck!).

What branch of the military? I was doing some consulting work for the Navy 89-91 during the first Persian Gulf war. Mainly writing software for compression of meteorological images but I helped out a bit on another project detecting transient signals in the underwater acoustical domain.

This was back in 2000 at Soto Cono Airbase Honduras, before that I was active army.