The ability to collect payment information right in the proposal will cut down the number of steps a client will have to do to get signed up. If we can add a stripe (or other payment method) integration right into the proposal builder, then the client can add his credit card details, sign the proposal and press submit. We're then not faced with the added work which it creates to get a proposal signed, then create an invoice and send that to the client and then wait for him to pay the invoice. Adding the payment into the proposal will shorten the process and get it all done in one step.