The problem with shields on capships is two-fold:
1) To make a skin-tight shield-grid (the diablo had a 'relative form'), youd need

of polys, effectivly doubling the poly-count on the ship. It would also be very difficult. Therefor, a bomber could slip between the gap in the shields (since these are plasmic shields, not force shields), and launch its warhead, then escape back outside the shield, where the shockwave would reflect off it.
2) The main threat to capships besides bombers is beams. Currently shields do not deflect beams... so much for the shields.
However, the AI can handle shields fine.