Most users have a phone already connected to their HA user account anyway, but only for presence detection. It would be great to be able to mark that phone also as the default notification channel for a person. That would simplify building automations:
Instead of “if this and that happens, send a message to notifier.xyz” it would just be “if this and that happens, notify Peter”.
Less hassle for experienced users, easier entry barrier to create automations for unexperienced users who don’t know about the underlying concept of notifiers.