I don't know of anyone who has objected in our school, and I can well imagine it being a pita for the staff if they did, although I can understand some of the reasons for the refusal.
Met a foster mother on a recent cub trip who was happy for the foster child (from a different group) to be photographed within the group photos, but specified what they could be used for and not used for. Good understanding all round, but if she'd said no to any photos that would have been fair enough imo.
If a parent does object, then I think it's acceptable for a member of staff to ask them why, or to talk through the reasons why, although I agree it should take place privately.
By talking it through, the parent may change their mind if a member of staff has taken the time to explain what the photos are used for and why it can be difficult if a child is excluded in this way (without labouring the point.)
Of course, it may just make the parent more adamant in their refusal.