Many people would like a Pro account, but would not like to keep their payment information online. This can be either due to security reasons or (like me) because the payment is made by the employer and the employer does not want to put his payment details in the worker's account.
Currently, the only way to buy a Pro account is to put payment details in the account, and they must remain there forever - there is no way to remove them.
I suggest allowing users to make a one-time payment (e.g. for a year of Pro account), and then let them extend the plan manually for another year, without keeping their payment details.