In patch 3 of this series you can see core of the issue, applications have no way of finding if the transport layer under RPMsg has a maximum supported message size. The examples end up having to look directly into the transport layer to find this info. With this API they can ask the RPMsg layer without having to know about the backing transport layer. This is better API layering and allows the core of the examples to work across any transport layer.
In patch 3 of this series you can see core of the issue, applications have no way of finding if the transport layer under RPMsg has a maximum supported message size. The examples end up having to look directly into the transport layer to find this info. With this API they can ask the RPMsg layer without having to know about the backing transport layer. This is better API layering and allows the core of the examples to work across any transport layer.