Planetary Shield Generators are borderline worthless.  If your enemy blindsides you with a massive force, they'll kill the shield generator anyways.  If it's just a small raiding party, you'd have been better off building other types of defenses to kill the raiding units.  There's really no purpose for shield generators.

You can defeat the AI siege frigate raiders more easily and cheaply by spending that money on turrets or hangers.  Heck, I usually just leave ten or so scouts on defensive duty and that cleans them up just fine.  You'd have a point if the shield generator were cheap or available early in the tech tree, but it isn't.  It's a high tech structure that's overpriced for what it delivers.

I haven't played against your super siege frigates; they might actually give the shield generator a purpose, but such a unit doesn't exist in the real game.  If it were possible to cheaply hit a planet for a massive amount of bombard damage in a short period of time, then the shield generator would be quite worthwhile.  As it stands, however, orbital structures are arguably far more vulnerable than the planet itself to harassers.  Bombard capabilities are generally expensive and inflexible, so it's much easier to target your harassment on orbital structures.

While you could build a dense formation of static defense around the shield generator (obviously how it was meant to be used) this has two big issues.  First of all, static defense in general is overpriced and the starbase is really the only thing that gives you good bang for your buck.  The second problem is that the starbase itself has an upgrade called "auxiliary government" that makes it so your planet cannot be lost due to bombardment.  While you can still lose population (and therefor tax income) this is a far more reliable alternative to shield generators, and to take it offline you have to kill the starbase.