I doubt very much if anyone uses it. I've never used, don't see the point

The SG gives 30%/50%/80% planet bombing mitigation (the additional mitigation is acquired through a Tier 7 research subject). It's a straight reduction in bombing damage taken (so with 80% mitigation, sieges, capitals, Novaliths, and presumably Meteor Storm will only have one-fifth of their normal damage - it would be as if a 6k HP planet had 30k HP).

TEC Shield generator is the weakest of the "Super" defensive structures. Advent have the AM recharger, which reloads AM on ships and defensive structures, Vasari have Nano Weapon Jammer, which disables weapons on attacking ships, TEC have the shield generator which, against AI is Ok, but is utterly useless against human players. I have been playing MP for over a year and have never once seen it used, because its effects can be negated if its destroyed. At least with Advent and Vasari, they offer assistance to other defensive structures and fleets, TEC shield generator only protects planets until they are inevitably destroyed, and there is a earlier tech that does a similar job, but better. In entrenchment, starbases can be upgraded to prevent the loss of the planet they orbit to bombardment, until the starbase is destroyed. And starbases are much more difficult to destroy than shield generators and come at a much lower research tier.

The shield generator is practically worthless unless you're blocking Novaliths.  The problem is that it's very easily destroyed.  To be useful, a shield generator requires a very specific situation:  the enemy has to be able to attack the planet, but for some reason be unable to destroy the orbital structures.  This is an unusual situation to begin with, as most of the time the opposite is true; it's much easier to destroy the orbital structures than it is to attack the planet.  The only realistic case I can think of is the Novalith.

You could argue that the starbase could protect a shield generator, but this has its own issue.  Starbases have an upgrade available that allows them to maintain control over the planet even when it's bombed out.  This is far more effective than a shield generator, effectively granting IMMUNITY to bombardment and requiring you to destroy a full-sized starbase rather than a little generator.

The bottom line is that you can safely ignore this structure.  I've never used it or any of the other "super defensive" structures in multiplayer.  Repair bays, phase stabilizers, phase jump inhibitors and occasionally hangers and turrets are the only worthwhile tactical defenses IMO.

There have been several suggestions.

- Cykur proposed having it also project a shield around neighboring structures to protect them.

- I proposed an idea whereby the additional safety granted by the shield generator attracts more people throughout the empire to come live there.  BONUS EFFECT: higher population.

- I proposed another idea where the tec novalith would be buffed to be able to remove control over a planet even with the starbase upgrade.  However, if you also had a shield generator, you would be immune to this.

I think a few other people have made other suggestions.

What about a 5% etc. increase in alleigence for planets protected by a shield generator?  You are spending money to protect the planet's inhabitants after all - unlike like the starbase upgrade which just maintains the idea that the planet belongs to you, and ignores the actual genocide of the matter - so it figures that they would have a greater loyalty to your empire.

I don't know Dorian, that seems like an awful lot of buffs (or are you saying only one of them is necissary?). I think I might have to rule that shield generator OP, even if no one would ever rush to get them. Furthermore, its currently impossible to make a buff target cruisers and not frigates.