I have had extensive debates with the author of the Spada paper on the meirts of using duelling shields to extrapolate the proper use of the medieval kite or round shield. A link to one such;
http://netsword.com/ubb/Forum3/HTML/000967.html
It is my position that the duelling shield is a specialized weapon that requires specialized tactics. It is also my position that the shield forward position protects more lines from attack at one time with only a small shift of the sheild. It is my position that the sweep from inside to outside ward leaves you slower than you should be in covering and very vulnerable to vertical cuts from a feint.
In the end, we must all speculate concerning what the true
manner of using the sword and shield in personal combat was in period. There are no manuals to clearly point the way. I do not discount the Spada article altogether, but neither do my experiences and research allow me to agree with it's conclusions. In the end though, in the absense of proof(there is evidence for either position) we're all making educated guesses in this particular realm of WMA.