Slide open phone, one touch to open app, one touch to take picture, as few touches as possible to re-engage 2nd party "user" of app.
Multiple apps to see what the pic looks like on my wall? Multiple apps to post snarky comment about pic on my friend's timeline? May be you are right that this is a use-case tech people have but from my observation, I see this pattern repeated all the time on facebook activity of non-tech friends.
On more than one occasion I have elected to use WhatsApp over facebook Chat app because of the app switch. My thinking is "hey if I have to switch apps, might as well pick the better chat app of the two". If this was built into the main FB app like it used to be, I'd be less likely to use WhatsApp.