Hopefully rubbing on the bumper is not a factor when going over rocks.
The Jeep bumper is to protect against light bumps and to help with mounting lights and a winch. A really heavy steel bumper attached to a steel frame will not help much for a significant bump since the stress will be transmitted somewhere else in the Jeep (including to the humans inside) where the damage is more expensive to repair.
It is true that 30lbs is not much. It all adds up.
I know someone with a JL Sport who rolled the Jeep into a tree, ruining the light bumper he had installed (the rest of the Jeep was fine). So then he installed a super-stout bumper with the Harbor Freight Badlands winch. Now the Jeep has a serious case of front-end droop (over 1-1/2") which seems at the limit of what leveling kits can fix.
Perhaps it is a philosophical thing but it seems like a good philosophy to only add weight when the benefits exceed the costs.