Paying makes sense when you actually need/use those services. Paying "because you can" feels wrong to me. If the goal is to support FOSS, there are many more ways to contribute than subscribing to a service you don't use.
Without commenting on Ubuntu Pro specifically, the whole point of a Linux distribution is that users don't need to know or care what specific services they use. I'd happily pay $50/year to use "Debian" and trust that the Debian Foundation figures out how to feed that money back to the appropriate upstream projects.
Please don't try and make me give $0.05/week to dbus maintainers or whoever.
Please don't try and make me give $0.05/week to dbus maintainers or whoever.