The problem (my opinion, not necessarily the op's) is using the cell phone number as the public id. An alternative id would promote better privacy. If Signal scales to even a fraction of the user base of a Facebook or WhatsApp, it needs to reconsider using the cell phone number as a public id. I can see where a group invites you to join and there's a bad egg in the group. That bad egg (randomly or intentionally) uses your cell phone number for all sorts of nefarious intents. I like to keep my cell phone private and not have it out in the wild.