I would issue a challenge to those who don't carry shelter (garbage bag, poncho, space blanket etc) as to whether they think in a real crisis they could get a fire and a debris shelter together readily?
Watch the hypothermia segment of this season's premire of Bear's show and remember that an ounce of prevention is worth a tonne of cure, and that every weapon in your arsenal might help in the fight. Carry both.
Heat sheet, AMK bivvy, mylar blanket, poncho... lots of lighweight, small space, inexpensive opitions for shelter.
Firesteel, lighter, matches, sparklight, cotton balls, quick tinder, dryer lint, esbit tabs, candles ... lots of lighweight, small space, inexpensive opitions for fire.