You are so very right. A heat sheet helps but it does not provide the warmth of a sleeping bag. The only problem is that a sleeping bag (at least mine for that matter) is that it is as big as my entire kit!!

My bug out plan does not include sleeping in the wilderness without shelter.

I am not discrediting the idea of carrying a sleeping bag in your BOB (in fact i am thinking of strapping mine to my BOB right now!!) but for some users, it isn't their number one priority for BOB contents.

Just my $0.02