Nobody costs extra, that is what no seats means. Contractors, interns, advisors, the whole company works from the same brain at the same flat base. The marginal cost of including a person is zero, so include everyone.
Seat pricing quietly shapes who gets knowledge: companies ration licenses, and the intern who most needs context is the last to get it. Removing seats removes the rationing and the shadow-account workarounds with it.
Access control still applies individually, permissions and visibility follow identity, so 'everyone is included' never means 'everyone sees everything.'