A shield slung on the back sounds like it would reinforce the armour of the man, but as arrows and bolts penetrate a few inches into the shield, it would seem like a shield/pavise held away from the body would be more effective against missiles. This is what my intuition would say, I've nothing to back the notion. No idea about the game mechanics of pavises, though.

Still, I would've loved to see pavises being placed on the ground supported by spikes, and having the Pavise Crossbows have to gather them again as the enemy cavalry approaches. They're not hindered by the pavises now, if I'm correct? I remember them being dead slow in MTW, and I can imagine carrying one of those shields along with a crossbow a chore.