this is exactly how it functions. it basically turns your shield into off-hand weapon. you do need more feats for it, though. shield bash, TWF line, double slice and shield master. +bashing finish, so crit feats(in case of paladin you can use keen from bonded weapon) as well and good crit range weapon. It's a big commitment.

heavy shields are -4 penalty and light are -2

So light shields is the same as having a light weapon in the offhand then, interesting. :)
Should be pretty neat with the vendor +5 light shield especially made for bashing that is sold in Pitax. :)

All things considered it's not really that good due to special effects weapons can have. As far as I'm concerned, it's best used for sneak attack builds as those don't care that much for bonuses and do care a lot for number of attacks. Pure Vivisectionists come to mind. Not sure about paladins and rangers as their AC sucks either way, unless you go a mile to fix it, so they might as well use 2 weapons instead. I guess rangers have eaglesoul, so that's something.