May I ask, why? With all the other options, including just text messaging?

Good for you, and all other Messenger-only-users. But why force all other users who are using the Facebook-app to open up a separate app when they want to send or read messages?

They expected to add a TON of complexity to the messenger product (they announced messenger as a platform at F8) and they needed a way to decouple the already complex main FB app. It buys them a bunch of things, like easier maintenance, extensibility, and ability to push updates independently among other user experience benefits.

They also didn't want to split the user experience of using Facebook messenger on mobile and hence the forced switch.

It was only a big deal because of the surface area of the world that the main Facebook product (w/ messenger) covers. I guarantee you that if Google did something like that no one would care (at least not enough to give 20,000+ 1 star reviews)

Yeah I'm one of the messenger-only people, and (obviously) I'm really happy I can use the messenger-only app, but, wow, the way they handled the integration w/ the main FB app is ungodly inelegant, almost to the level of being insulting to the user, IMO.

I wouldn't call "easier maintenance" a "user experience benefit".

This reminds me of the talk "Is it really Complex? Or did we just make it Complicated?" by Alan Kay.

The complexity of the underlying problem, messaging, has not changed, but FB has made it a lot more complicated.