I think the rocks in Steppes of War have no purpose other than to be annoying. If they attack from the lowground, or from the highground blocked off by the rock, you defend from the same choke. You must destroy the rocks for your 3rd to be safe. It doesn't delay your expansion at all because you can run a worker around the long way to build before the rocks are fully dead.
edit: i'm totally not talking about the yellow mineral rocks.
Edited by LysIs514, 22 March 2010 - 01:02 AM.