ALOT of apps are starting to do this. I believe Snapchat initially was the company that began doing it. When a site like Facebook uses your number to identify you, this feature actually makes sense.
I wonder if more fine-grained permissions would make more sense for this sort of feature. Perhaps it would be better if Android had support for "This app may read text messages from 1-800-XXX-XXXX" or something similar instead.