Feature bloat is one of the worst things for a security conscious product. The more features, the more attack vectors. Nobody asked for payments in Signal. Where did this idea come from? It was never put forward prior to this, it was never on the road map. That only makes me even more suspicious of this decision, which leads to my second point...
MobileCoin has all the appearances of a scam. 85% of the coin is owned by the creators. The price rapidly shot up at the end of March. The social media of the developers was posting rubbish for a long period of time. There was no mention of this collaboration beforehand. This has all the hallmarks of a pump and dump. Have the Signal devs been duped? Or are they wanting to cash in on Signal's rising popularity?
Anybody at all with an interest in Signal needs to let the foundation know that this Beta needs to be scrapped, and that payments should never be added.