OP, it sounds like it doesn't get cold enough where you live to justify using it. I have driven diesels all my life, with the exception of my current truck, and believe me when it gets cold, you want a bra.
I remember when we got an early cold snap one year, and I made an hour long drive, and my truck got colder the further I went. It started the morning in a heated shop where I idled for a couple minutes before pulling it outside. The heater was kind of working, and I was warmer in the truck than outside. After about ten minutes on the road I realized the temp guage was at the bottom/below the "normal operating range" marker. My toes started to get really cold in my whites packer boots lol, and by the time I got where I was going my ass was about frozen to the seat.
Granted this was around -50, but a bra would have made a huge difference considering the truck was cooling down because of the highway speeds, and very cold air. I drove home in 5th gear instead of 6th, and went about 50 instead of 75-80. I put my normal cardboard insert in that night.
You probably don't NEED one, but I won't hesitate to put mine on the ranch's 2011 ctd 3500 when it starts to get cold.