As an individual, no, there's no need, you can use your phone number as a user ID (although this comes up as a concern whenever Signal is mentioned as an alternative). There's also a practical factor - using device IDs allows end-to-end encryption to multiple devices without the user needing to manage keys themselves. (ie: WhatsApp can just create a new key pair for each device).

As Facebook however, there is a business need to be able to identify a single user across multiple devices in order to properly track them for marketing reasons.

If Facebook are unable to do this, then the business case for operating a free messaging service falls apart. Each individual needs to make their own decision as to whether they are prepared to pay the price of sharing the data that Facebook collects in exchange for simple, free-of-cost, international messaging. It's not an easy decision.