Go ahead and download Facebook Messenger. I enjoy using it. But you don't have to. If you have it downloaded, messages are available in the Facebook app. So you could just hide Messenger in a folder with other unused apps, like Newsstand. It will take a bit of space though. About 60 mb. But you would have messages in your Facebook app, and no more requests to get Messenger.
I'm with Mr. Steplin on this one, I rather enjoy using it and have been long before they decided to make it mandatory. Give it a try to see if you might like it.