Actually the messenger app is superior to the native Facebook app's messages section. Also it takes no additional time to get into the messages app than the messenger section. I don't trust App Store reviews due to trolling. I trust personal experience only.
Having the messages app separate means that the team working on it can improve it and fix it and roll updates out when they need, separate from the main app. Would you prefer they didn't do that? That we would have to wait until the main app got an update to then allow messages to be updated too?
Making stuff up to try and support your point there about needing to have phone number to properly work. You do not need to have a phone number to use the messages app, I've never put my number into the app and I've never allowed phone book syncing and it works fine. Putting a phone number is is optional.
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I just compared it on an iPhone 5S. Same amount of time to get into the messenger app as the messages section. No blank screens. Plus I'm even running it on iOS 8 beta. Additionally if I want to message someone I go straight into the messenger app rather than open Facebook, then tap messages. One less step, freeing up more time....