Shield are in fact to static, no active or passive skills which only affect shields. The only skill that mentions shields is "Intercepting Guard" and works just fine without it

Im currently testing theatric with shield in BG and all Kreegan said is true. (shadow&myth school working around shield user, Rages and some Wars ignoring your shield)

If we are in dreamland territory, I would wish some active skills that work only with shields. Something like "Entangle", we need just one skill with different effects for each size of shield.

Something like:

"Holding you Ground"
Gladiators fighting with a "Wall Shield" will be able to withstand the mightiest attacks, absorbing all damage done by "activated skills".
Gladiators fighting with a "Buckler" learned to use the momentum of incomming attacks to redirect the blow just enough to avoid certain death and counter with a quick answer.
Gladiators fighting with a "Kite Shield" know how and when to switch out of defense into offense, surprising the opponent with a shieldbash.

Detail:
Wallshield works like "uncanny dodge", only on activated skills to balance it and 2-3 turn cooldown. Very defensive.

Kiteshield works similar to "riposte", only activate when you parry/block an attack and dealing a medium amount of damage back. Maybe 2-3 turn cooldown. Balanced between offense and defense.

Buckler could work like counterattack, only activate when you parry/block an attack and dealing an additional attack that turn based on the number of parrys/blocks, maybe only extra vanilla attacks to balance it with 1-2 turn cooldown. More offensive than the bigger shields to justify the low base defense.