I can't think of any plugins that have server and pro versions that work in concert like you describe. Do you know of any that you have in mind, or are you speculating on how they would be used?
In your scenario, it sounds like it would just add latency to pass a SQL command to the server, which then passes it along to another SQL backend get results then hand back to a Pro plugin... just adds one extra "hop".
Nevertheless that is how the benefits of server side plugins are advertised.
I don't see how it adds a hop. You have to get the data from the server in the first place; and either you get all the data and then parse it at the client; or the server sends only the limited data that matches the query.
Server-side plugins are there for the purpose of allowing server-side scripts to use them (either by schedules or when called through the XML or PHP API). FMP or its plugins are not involved at all here.
FMS *can* evaulate certain field calcs server-side instead of having them performed at the client. This scenario is the only one that comes close to what you describe. And for that scenario you would indeed need to have the plugin both on the client and on the server. In case FMS decides not to perform the calc before sending the data, the client will need to resolve the calc.
If what you have in mind are scripts then you are out of luck, scripts are always done by the client (except in schedules and API calls).
Thanks very much for the clarification. It isn't so much what I have in mind. I am just trying to clarify a fine point about server-side plugin behavior.