In a spec like the tank or tank hybrid, 23/1/17, wouldn't you have the following abilities that benefit from a shield?:
Lightning Reflexes + Lightning Recovery: this gives you back 4% of your total force AND reduces the cooldown of Force Shroud by 1.0 sec every time you shield an attack. (Considering the low amount of attacks that will proc your shield, are these even really noticeably beneficial at all?)
Dark Charge: this gives you +15% to your shield chance.
Dark Ward: this gives you another +15% to your shield chance.
Hollow: this increases your shield absorption by 4%.
So the math would probably look more like this assuming you have Dark Charge AND Dark Ward up:
35% shield chance x 24% absorb = 8.4% mitigation of white damage attacks.
That's not too shabby, but not too great either. I think it mostly depends on your style of play. Are you the type who throws guard around a lot? Use a shield. Run the Huttball often? Shield. Re-modded all your gear to maximize power and like to dps? Forget the shield and continue to max power with a focus.
I currently wear a shield in pvp with the 24/1/17 spec. I have tried both. There isn't much of a difference really. A bit more damage mitigation versus a bit more damage output. Neither will make or break your pvp experience as far as I can tell.
Read the tooltip again. Shield, block or parry. So even without a Shield equip you still get benefits from the talent.