And people are terrible friends and will take the time/effort to bypass any measures necessary to share the photo.
The feature is designed to address the "somewhat bad" case, not to be perfect.
Exactly. They will pretty much bypass this one as well.
Reducing the chance of something that you didn't want shared from being shared by a factor of X% is better than nothing. Try to step out of black-and-white thinking for a minute. Just because something is not perfect doesn't mean it's useless.
Yes, your example of trying to prevent someone moderately bad from sharing something personal to you which you don't want shared is reasonable. The trouble is, you can use the same mechanism for much less reasonable and more nefarious, corporate things.
So the end result is that we're not to have moderately nice things, lest they be used as a weapon against user freedom by a mega-corporation.