Since Host and Viewer are not bundled together and if backward compatibility breaks or changes on every version, then it should be more obvious to the user when there is a version mismatch that would prevent connection.
Newer Viewer can always connect to older Hosts. Compatibility the other way around is not guaranteed. That's why we highly recommend to at least make sure the Viewer version is the latest if it's not immediately possible to update the Hosts.
The Viewer knows the version of itself.
This is true.
The Viewer knows the version of the host.
Only when it connects or logs in on the Host at least once. If that happens, information about Host version becomes available in the address book. In the thumbnails view there is a tiny "msi" icon at the top right corner. In the Details view there are two dedicated columns:
The typical user (*cough* *cough*, myself included here. Though in this case, I KNOW I've read that before, just forgot) doesn't read documentation until they run into a problem. Errors and messages help tell the user where they should look in the documentation. The pop up informs the user what the exact problem is and what to do about it. Problem solved in minutes.
I can agree with this, but unfortunately, when version mismatch is involved there may be a problem or there may not. It's a probability thing depending on what specific feature the user is trying to use. Less intrusive methods of informing about version mismatch (such as the ones above) may work best especially on hundreds and thousands of address book records.