It used to cost. It is now free so that helps the value proposition. There will be a Beeper Pro/Plus or some subsctiption service that will allow you to have multiple accounts/bridges on the same network, but that's not actually released yet.
Curious about this as well. Since Beeper uses bridges to communicate with other networks, (Signal, Whatsapp, Facebook, etc) and it needs to decrypt the messages before sending them over these bridges it is inherently less secure than using something like signal directly.