Not all have sametime enabled. Using the screen capture tool is often done within the Notes client.
I suggest to move the tool from Sametime plugin to the Notes client. (or whatever you would call it)
Currently you must be logged into Sametime in order to use the screen capture tool. It should be available at all times.
The idea is trying to improve notebook ( new feature in 8.5 release , formerly known as Journal ) utility. The notebook is used mainly to document and store general ideas, attachments, documents, etc.
When you are working outside Lotus Notes, maybe using the browser, or MS Office or ...