Shivan small ships are essentially disposable. The Lucifer is (as far as we know) a one-of-a-kind craft. It only makes sense that, were the Shivans able to shield something in subspace, they'd put that effort toward the Lucifer and not a few dinky Scorpions. And that's completely ignoring the point that, as far as the GTA and PVN were concerned, the Lucifer's shield was fundamentally different from any others out there, a quirk which may have enabled it to work in subspace.
Was it really fundamentally different, though? The size of the shield generator on the Lucifer must either be huge or of capship-integrity, in which case the Lucifer would be able to regenerate shields at a much faster rate than all the firepower of the GTA ad PVN combined against one quadrant, like what Woolie Wool said.
The thing is that the shields only protect the craft from energy- and missile-esque damage, not huge collisions. The Ancients must have encountered something like the Lucifer, otherwise they would not be extinguished. Hence, what the PVN found on Altair really was the key to destroying the Lucifer.
If the Ancients were being threatened by small craft that they couldn't destroy, they could ram them, panic over. However, if the Ancients were being threatened by large ships that they couldn't destroy, ramming wouldn't be an option as they would be mowed down before they can carry out their suicide.
* Androgeos Exeunt is wondering if he is lost. He thinks it is highly probable.
