Except in any actual shieldwall situation (like siege defense), the +shield skill is useless because some mauler will just come up and crushthrough your shield.
The other day I had +5 shield from shieldwall (for 10 total skill), and a mauler just ran up and crushed me, since crushthrough is only a function of shield weight and not skill. Silly that 1 mauler can negate 6 shielders working in unison.