Just wanted to provide an update on this.
Overall, our position on mixing free and paid agents did not change, and is not expected to change for the reason I've explained earlier in this thread.
However, as of Update 4, all sockets licenses include a few gifted instances: 1 per socket, maximum of 6. Depending on the number of sockets and your product edition, this should be enough for most customers to cover all their free agents (see conversion table
When you upgrade to Update 4, you will note there is the new check box in the license information dialog saying "Allow free agents to consume instances". If you flip it on, you free agents will automatically consume licenses, making your entire deployment including those agents fully supported.
Additionally, you will now be able to manage your agents centrally by creating the corresponding agent-based jobs directly in B&R console. This will use the same number of gifted instances as with "standalone" agents pointed to a backup repository, while simplifying your agent-based backup management dramatically.