One of the best features of Signal, and one that massively helps adoption with the less tech-savvy crowd, is that you can set it as the default SMS app on Android, and it then uses Signal for contacts with Signal.
If you can't tell if a contact has Signal, it would have to default to SMS - and when sending a Signal message (to either a phone number, or in the future, a non-phone identifier), there'd be no way to tell if you're sending it to someone with Signal, or sending it into the void.
Maybe that's a trade-off you'd be willing to make, I don't think it's cut-and-dry though.
Not saying it is cut-and-dry, but atm the users doesn't get to make that tradeoff for themselves - signal made it on their behalf when it could have allowed the users to choose on activation to which of their contacts they wish to be discoverable.
