Lance,
Your comment about the cars coming home from OUSCI into cold weather reminded me of an experience I had with my Camaro shortly after I purchased it in 1973. I was attending school in Phoenix at the time and I had filled the radiator with straight water since it never got cold there. I headed home to Kansas for Christmas and by the time I reached Flagstaff the heater was no longer blowing even warm air. I stopped at a service station to find out why and after a quick look around found ice in the radiator. I was able to pull the car into the station's garage where we drained the cooling system and refilled it with proper anti-freeze. I have never run straight water since.

Switching back and forth between water and anti-freeze will be a real pain. Unless there is a specific rule that states I have to run water only, I will stay with the 50/50 pre-mix.