Consider example scenario:
1. I want to connect to VNC server
2. tsclient has the protocol grayed out in the combo box
3. Have to perform research: why is it gray? Is it my fault? Wrong settings?
4. Install the missing packet.
What a waste of time!
Some apps do it right: just suggest installation of the necessary packages (see e.g. rhythmbox way of installing gstreamer plugins).
My proposal: do not make any protocol entries unavailable entries. Integrate with the environment to ask about installation of the missing pieces of software.
E.g. Ubuntu has xtightvncviewer and xnest packages.
Do not let many users rediscover the solution again and again.
At least: explain the matter somewhere, e.g. in the About box.