Definitely required.
If there are any active services then client should be required to have a valid/working payment method set as default.

This is how most billing systems work, they will allow you to add/remove payment methods, but there must always be at least one active.
So if you only have 1 card then you must add a payment method to delete it.

It would also help if when a customers pays an invoice manually, it would ask them if they want to make that the default payment method.
As it is also a common problem that when a customers card stops working, they login and pay manually, but the old (not working) card is still set as the default, so payments continue to fail.

Customers (especially computer illiterate ones) do not understand that they need to login, delete the old card and set the new one as the default.
This becomes more problematic if we have disabled the ability to delete payment methods so that they cannot end up with no payment method.