Sonno: How you will block area with RC/OM? Enemy team will destroy it then easily...
If you place the RC/OM infront of your base and can keep it alive before SD starts, you are good to go with that tactic, honestly i've had very few games where the enemy team actually kills the RC when I block with it, I haven't done that much with the OM though.
The idea is that the RC/OM is a centerpiece in the game. After sudden death, if you don't protect it, you will lose the game. I mean an option could be allowing rebuild of the RC/OM during sudden death, which I actually think was allowed in Trem. But, making those structures invulnerable really drags the game out.