I think as a family member you never ever want that for them, but when someone is in pain, or no longer the person they want to be then I can imagine people allowing them to do it.
Dementia is particularly hard because people stop being the person they were, but by that point they can't make the choice for themselves. The mid stages of dementia are heartbreaking for family, and scary and confusing for the person. The later stages are still heartbreaking for family, but the person is often no longer aware of what is happening.