Proponents of these type of systems argue that it is easier for a subscriber to learn the payment system as it is based on something that they are already used to. They would not need to download a specific application and can start usinng the system immediately. It is also ideal to build viral characteristics into the system as many people are already using BBM. Similar arguments are being used for the recently launched Google payment solution to be based on gMail. (Read here)
The more interesting challenge from my perspective would be to ensure that the registration process is secure, that the source of funds are well worked out (for instance will this service require robust cash-in eco-system, or will it be based on debit cards) and ensuring that subscribers will be confident in using a messaging system to do payments too. There is still a lot of work before these type of payment systems will become mainstream.