As for their apparent fragility in DS9, that seems to be a problem with even the largest of combat vessels in DS9. Small numbers of hits at the beginning of a battle, at a time when shields should have been at 100%, were sufficient to cause massive structural damage to Warbirds, Galaxies and pretty much any other big ship type shown in visuals. The allied attack to retake the Chintoka (sp?) system is the prime example. The visuals did make the Defiant look ever so much tougher, though.
I've had a theory that I thought out to explain this sudden fragiltiy of ships - not just federation but ALL trek ships in DS9 seem to take a few hitrs and their out of the picture...compare to just as recent as "Yesterday's Enterprise" when the E-D takes on three Klingon BOPs and they exchange fire for more than 5 minutes, continous fire and the shields slowly get battered down. Then look at episodes like Sacrifice of Angels where Fed ships take a few hits and they explode, or a Galor class cruiser's shields fail on the first strike of a Galaxy class' phasers.
Throughout military history it seems that there are two periods of ascendency, defensive tech and offensive tech. There are stretches when defenmsive technology is in ascendence and it takes a lot to overcome this advantage : Medieval knights were masters of the battle field for centuries, as the plate mail got heavier it got to the point where theywere simply invulnerable. Ascedncy of defensive techonology ie. armor.
Trek would have such a period during the TOS and early movie period, when shields are incredibly powerful and we have no exploding console syndrome or hull breaches, it takes ALOT of punishment to bring a ship's shields down. It would also explain such odd weapon development like the Romulan plasma torp, recognizing that they needed to find a way to bring down Fed shields quickly they devised these weapons, low rate of fire, energy intensive, but it overcame Fed shields easily.
Then we get into the assendency of offensive weapons: For us, that would be the advent of gunpowder and Napoleonic warfare where no body armor is used and the gun rules the battle field.
Here I would place DS9 era tech. The weapons systems of the new generation of ships is so powerful that the new standard shields simply cannot repel that kind of firepower, allowing for the quick kills we see on DS9.
Then there are odd periods of equilibrium where defensive and offensive tech are sort of at a par, MAYBE we can say now with the advent of kevlar that defensive tech is catching up with the gun....
For ST that would be TNG era, where the weapons are more lethal but the defensive systems are able to keep up.
Anyway, my long winded idea on why starships are blowing up left and right as if shields no longer exist on DS9.
