i assume that if your license doesnt include the sharepoint licenses then the plugin shouldnt work...?
I can only guess that your'e right. I'm looking at my services now, and I have 8 running and 8 installed. None of which are sharepoint.
Which brings me to the other question.
Sharepoint is just a website with data being stored in a back-end SQL server database. Why exactly would you need an actual "plug-in" just to scan sharepoint as you're accessing it? The web shield would scan the page as soon as you access it, files would be scanned with the resident scanner as you downloaded or uploaded them. The only reason a "plugin" would be needed (in my thoughts anyway) would be for the server, so that it can scan the database as things are changed.
I do understand what you mean about the licensing though, even though you have licenses for outlook, doesn't mean you can have 10000 clients access exchange, without the appropriate 10000 exchange licenses as well.
I just don't get why you'd need to run the plugin on client's computers, that's all.