In my playtest, we have begun to use Shield bonus for several combat maneuver calculations. I have allowed the bonus to be added to CMB for bull rush attempts. I also allow Shields to add to CMC (combat maneuver class? or whatever the DC to pull of a maneuver is) vs. select maneuvers.
You might consider this as an additional incentive to use a shield; one that does not require a feat investment, etc.
Hmm, interesting.
Bull Rush Attack - I can see this
Bull Rush Defense - not so sure
Disarm Attack - nah
Disarm Defense - nah
Grapple Attack - no way
Grapple Defense - .. maybe
Overrun Attack - maybe
Overrun Defense - could see this
Sunder Attack - no
Sunder Defense - no
Trip Attack - no
Trip Defense - no
Not sure it would be worth the space in the rules for 2-4 items.
Still interesting.
Maybe combine this with the "Shield to Touch AC when on full defense"? Then I could see most/all of these.
Combat maneuvers are already a challenge at DC 15+CMB. I could see your suggestion working if it were set at 10+CMB. Then the shield might be an asset vs. CM attempts.