The purpose of adding a payment method to your Roku account is to allow easy payments via Roku. If you don’t like their payment system, you can just remove your payment method from your Roku account, and then pay individual content providers if/when needed.
I’ve done it both ways. At first, I didn’t give them a payment method. A few years later, I gave them one and it was used to bill exactly one thing for a few years. Then I dropped that service, and removed my payment method from Roku.
Either way works for me. Many people prefer to directly be a customer with each content provider anyway. That allows you to sign in on any platform, and if you want to complain to the content provider, then they know who you are.