What you say about puffing up the feathers to stay warmer, drier and to dry is right on.
I have not seen gobblers go into full strut in order to stay warm around my area of WI. While in full strut they can't eat or walk around all that well. I interpret the full strutting outside of mating as an act of aggression, show of strength to other toms. Normally they are not with any hens at this time of year nor are there any hens or poults in the vicinity.
You have me curious that I might have missed something I will pay more attention to details of the off season strutting activities.
If you agree with me call it fact; if you disagree - call it my opinion.
After all - we are talking turkey.