* You can use Signal with any phone number that can receive voice calls: http://support.whispersystems.org/hc/en-us/articles/21319067...
* Signal's primary audience is non-technical users. The big goal of Signal is to thwart passive, dragnet surveillance of people's text and voice messaging.
* AFAIK, Signal's use of GCM makes it more difficult for an adversary to perform traffic analysis to determine with whom a Signal user is speaking, but not impossible.
* If you don't trust Open Whisper Systems to treat the phone number that you give them as sensitive and confidential, you really can't trust their messaging software, either. :)
Agreed. It was even point number one in my list of commentary:
> * You can use Signal with any phone number that can receive voice calls: http://support.whispersystems.org/hc/en-us/articles/21319067... [0]
I heard a long time ago that the dependency on phone-number-as-username is something that they want to get rid of. However, I'm pretty sure they've been busy with the WhatsApp work and doing general work on the existing clients, so I don't know the timeframe for such a change.