With cross-play across all platforms, you and your friends can squad up no matter where you play — no Microsoft account required.

However, signing in with a Microsoft account unlocks full cross-platform functionality. It enables cross-progression, so your Campaign and multiplayer progress carries across devices. It also allows you to send invites and play with friends across platforms — like Xbox to PlayStation or Steam to Xbox.

Honestly, this is the way to handle the incorporation of platform-specific accounts; totally optional, but actually gives you a sensible benefit if you choose to go that route. Sony could learn a lot here.