From the Ecosystem page: "So, instead, Roughtime is only available for products that can be updated. The server lists have an explicit expiry time in them and we will actively seek to break clients that try to use old information in order to maintain ecosystem health. At the moment changing the hostname or port of a server is the easiest way to enforce this but we expect to add a per-server id in the future that clients would need to send in order to prove to the server that they have a current server list."

I see a slight chicken and egg problem here. This service is mostly needed to provide initial time synchronization to devices that have just powered on and don't know what time it is - but of course, since they don't know the time they have no way of checking the freshness of the server list. Which in practice means that a lot of devices will end up having to download a new server list every time they need to synchronize their time. That's a lot of load on the server providing the list, which effectively becomes a critical part of the Roughtime system that's hardcoded into the clients.