Seems like not only SFC 3, but also this board looses it's people. Well, I think I can answer your question Adam.
When Data ordered his first officer to set the photons up, the first officer (can't recall his name) replied with something like "but that won't even...." (I think it was about the power setting of the torpedo). This and the fact that obviously no real damage was caused on the Romulan ships makes up for the conclusion that Data's chosen Torpedo setting was only meant to illuminate the targets. And if you look closely on the impact of the torps, you will see that the energy moving over the Warbirds EXACTLY flows over the hull, while if the shields would have been active the impact would have looked more like hitting a bubble around the ship's hull, but not exactly matching it's form.
This would fit perfectly to the fact that Starfleet's mission was only to UNCOVER the Romulans and their connection to the Duras, not to seriously damage or even destroy them.
Remember, in the episode "The Defector" it is mentioned that both the Romulans and Starfleet can set their weapon's power and efficiency very precisely, (now put in my words) rangeing from being not more than a flare to maximum damage.
So in short, there was no error, just a very precise and low-power Torpedo setting.
Hope that helps,
- Future
P.S.: Please excuse my sloppy english, I hope I was able to explain everything in a understandable way.