hi,
i think everybody has different experiences about this subject,
and main reason for this differences is every model and every concept is different,
there some things that might effect this model relation thing imo,
- concept of the shot, (business, fitness, or nude?)
- model's identity, (male, female, married, experienced or not)
- model's profession, (is she/he a friend, is model a pro, is model someone who find the photographer and never had a shooting before)
in my experiences, whatever the concept is, if the model is a female, bringing his husband or boyfriend is not a good idea, neither the model, nor the photographer would be free at the shooting,
but in location shootings with more than 1 model, bringing someone who is experienced in photo shootings can save your life if the others are not experienced,