how to use pruneDevice for pmos4

In divaLVS rule file, I added "pruneDevice(MOS ("pmos4" "nmos4")) to prune
dummy transistors.

I mean that dummy transistors are transistors with D, G and S connected
altogether.

"Mos4"s with drain, source, gate and bulk connected altogether were pruned.
However, mos with only drain, source and gate connected altogether was not
pruned.

How can I prune those dummy transistors?

One more thing. Are there any other reasons we use dummy
transistors except "etching issue"?