Of course, this wouldn't give facebook a detailed map of their user's movements.
Of course, this is Facebook, which has been fighting with Google for the title of "most creepy, intrusive, privacy-destroying business model". With their ethics, the answer to "what would stop them" is obviously "nothing".
There is a significant PR difference - reporting the GPS location at regular intervals is currently "just part of the regular status update", where Facebook gets to act as a passive man-in-the-middle who records those updates. If sending the location data as seen in all user visible situations is a manually-triggered event, extra code to hand that data off at regular intervals only to Facebook wouldn't be able to hide under the assumption that "it's a feature". In an ideal world, this would be criminal access to unauthorized data. In the current environment of legalized spyware, we are back to "nothing" mentioned previously.
(The terrible granularity in the permission models in these environments is a huge problem. As it affects far more than just location data, that is a topic for another day)
Honestly, and maybe I'm in the minority, showing that information readily to my friends is MORE of a red flag than if Facebook solely had that data. Because let's be honest... they already have that data from my usage on the FB app (albeit at a lower frequency).
A lot of these things are questions I routinely get/ask in things like messenger. People with location on usually help us to coordinate things and the like.
I can understand the divisiveness, but I can also imagine people working on messenger (who probably use it a lot more than you or I) being on this, and genuinely believing it leads to a better user experience for most people (see read receipts).
If the FB developers think that's generally true, I'd say they are pretty incompetent. Even my parents, with just a couple dozen friends, have FB friends they have never met, let alone my teenage brother and his friends, who have hundreds of FB friends each.
This is usually not a problem, since they don't post sensitive information on their timeline, only in the chat to specific people.
Even when people are incredibly careful people do unwittingly post things that allow others to identify them. They may not reveal much in any individual post but when you combine a few together its a problem.
1 prime example is a friend who posted an invite to a house warming party then a week later posted from their holiday abroad...
According to the article, this feature is also enabled for group chats, where you converse with people who are not even your FB friends.
people working on messenger
I happen to be working on a mobile messenger [0], and I can see the benefit of transmitting user location. Still, I would never implement it as an on-by-default feature on all outgoing messages. Sending a location should always be an explicit action performed by the user, for battery preservation reasons as much as because of the privacy.
This solution probably isn't particularly novel, but it's probably better than carte blanche location access, in the opinion of most people.
My wife and I use this service and find it quite useful - it seems like a much more sane way to handle location sharing than to simply attach your exact location to every message you send by default.
Sadly, since it's buried in Google+, most people are probably unaware of it.