They look pretty vulnerable to nades, and due to the way they work it would be pretty hard to put something in place to stop that happening without destroying their functionality.
i remember in 1.3.2 you could hold a block w/ the gravity gun right before fight, and during fight you could go through it and it would be transparent, but others couldnt go through it.
what i did was i used it as a personal gate, and i secured the block so enemies couldnt move the block out of the way. it was awesome. like you could shoot out of it, but everyone els couldnt, so i would shoot them, but they couldnt hit me back. it was awesome.