Hmmm.
When I suggested a "shield skill" I had actually considered the implications of actually wearing a shield on an arm as well as using it, since the RAW don't apply any real penalties (apart from Encumberance) on the ability of characters to do things...which for the larger shields, I think would be quite substantial. So for these purposes, I'd have made the "shield skill" an armour skill, with the ability to go past simply cancelling a penalty. Once the penalty has been overcome the remainder becomes potential "OB"
I.e. In order to actually use the shield for anything other than the purpose of an obstacle, the weilder would have to develop the skill to a point where any penalty had been imposed to it's use as a skill had been nullified.
Yes, regardless of the existance of a skill to use it, whichever hand it is in, the shield WILL provide an element of DB to the wielder.. in much the same manner as standing behind wall. So, in most cases, the character will recieve the base DB benefit, regardless of thier skill with it's active use.
Now, if the character is disarmed(perhaps even having his primary arm lopped off), no spare weapons to draw/recover and only has the shield to use... what would the character do? My guess, this is where the "skill" would come in, and thus also where the "off-hand penalty" would also kick in when he tries to use it.
Up to that point the character should have the option to either use the weapon as normal, or the shield actively, or attempt to do both (which is where the problem lies..).
I would suggest that the shields actually cause a MM penalty equal to the (melee) DB provided, make the Shield Category the same points cost as developing medium armour. AG/ST/ST. The various shield types are treated as different skills.