Let's do a comparison:
Off-Hand Parry + Buckler= +3 AC + enhancement bonus of buckler (someone forgot that buckler can be enhanced- oops!)
Large Shield= +2 AC + enhancement bonus of large shield
So, whilst used, Off-Hand Parry is clearly superior to the 'next best option', and better than Dodge (which only gives +1 AC vs. one opponent)
There are also several advantages when compared with Shield Basher:
Weapon can be enhanced to increase damage; Shield cannot. This also means Shield Basher is hamstrung against foes with damage reduction.
OHP wielder has a better arcane failure and armour check penalty than Shield Basher.
OHP wielder is more subtle than Shield Basher (see Silveraxe)
OHP wielder can be a viable Duelist; Shield Basher cannot (see Silveraxe again)
Shield Basher cannot use weapon feats, as a Shield is not technically a weapon.
In other words, Shield Basher has several problems. The irony is that Shield Basher, if attacking, receives a -2 to hit. OHP man does not, as he is not actually using his off-hand weapon if using OHP. So, if anything is trumped by Expertise, it is Shield Basher. Of course, if Shield Basher chooses not to Shield Bash, then he is inferior to OHP man in terms of defence. If he chooses to Shield Bash, the -2 to hit probably means he would be better off simply going for standard sword-and-shield with Power Attack. Offensively, he is clearly inferior to the two-weapon attacker due to the two-weapon attacker's ability to use weapon feats with his off-hand weapon and to have magic weapons- the +1 AC (as compared with buckler) is not worthwhile.