It seems to me that the sensible thing to consider in a survival situation is that you would want to hold your liquid past the initial urge, but not to the point of excessive discomfort or distress. Heat loss is a practical consideration, as is a full bladder will tend to diminish the production of urine by the kidneys slightly, thereby having some positive effect on reducing dehydration, but this is a marginal consideration at best.

On my first big hike in the Canadian fjords, I was able to refrain from defecating for three days with reasonable comfort. I don't know what benefit that was for me, other than conserving the TP, and possibly scaring a bear when I did finally seek relief. You can find a purpose in all things I guess.