Of course it is. Even if Apple can't see the content of your messages, there is plenty of metadata that they can collect to understand how people are using the service, such as e.g. the number of messages people send, to how many distinct recipients, if they are using continuity to send/receive messages from a Mac, and many other things.
Hopefully they use the differential privacy methods they bragged about at WWDC to truly anonymize all that data. If they didn't, messaging metadata could also be used to build social graphs a la Facebook, which has a lot of privacy implications. In many ways, metadata is more important than the content of communications when it comes to analytics.