Martin,
It's knotty problem that I don't have a final answer for. There are all sorts of nuances - not simply the shelter. I seek a layered, modular approach to the whole "essentials" kit that ranges from what's on my person thru day-pack level to full-up expedition kit. Somewhere in the vast middle area are the kits I would call my BoBs.
I don't include a tent in any BoB pack. That's add-on gear for us in the BoB realm - but from separable FAK on up there is *something* stowed in the shelter category. Temperate zone here with all 4 seasons from hot and humid to frigid and dry.
Ahhh - I'm not sure I'm being clear...
I'd like to pick this thread back up Sunday PM - helping with homework right now and I have to pack up some kit for a weekend outing afterwards.
Regards,
Tom