For Exchange client access servers you need to look elsewhere, Windows Performance monitor. From here we can easily get a telly of the number of users connected to our Exchange client access server. You can see in my lab environment I currently have 2 users connected through RpcClientAccess (Direct RPC or RPC over HTTPS) and no Exchange OWA users.
To get to performance monitor from a run prompt, type "perfmon" then add the counters as in the screenshot below.
Looking at the user count on each CAS server can be useful especially when you have a bunch of CAS servers in a farm with load balancers in place, you want to make sure the users are being distributed evenly across your CAS servers.
If you want to encorporate these performance monitors into PowerShell code, this is also achievable and has already been done. For more information on this please refer to the following blog posts: