So, servers are able to use this to capture your display and see what you see, but there is a severer problem here.
They can view the steam overlay.
Screengrab is a complete breach of everyone's privacy, do not give admins the satisfaction of banning someone due to this method.
Sure, screengrab crashes sometimes, but seriously, do not let them keep on doing this.
Here is a forum post from another site with more discussion
https://www.seriousgmod.com/threads/screengrab-stop-hackers.16241/