Skaven Hero Movement and Shields(?)

So do Skaven heroes (except the Gray Seer) really get 6 movement? What's up with that?

And why can't Skaven use shields?

1. Yes. They are trained assassins.
2. Some of them can. The ones who can't are meant to be lightly armed skirmishers/assassins. Ever tried climbing anything (let alone a drainpipe) with a shield on your arm? Even slung on your back it's difficult as there is a lot of weight which throws out your centre of balance.

As catferret said you can't really climb anything while using a shield (unless youve got really long fingers)
Shields aren't that sling worthy to begin with unless constructed for the purpose of carrying it on the back when not in use,(although it can be done) but the running point I think is that skaven are quite small creatures, Better suited for the high speed low drag approach of ninja-esque guerilla warfare.