In the late game, once I have effectively unlimited resources, I have a tendency to put a Vasari starbase around every planet of the races I have Cease Fires with (I never make actual Alliances, as it breaks all of the vision/ceasefire/trade pacts when they fail). I then give each a phase stabilizer and lots of combat upgrades (2/2 weapon, at least 3/4 health, and 1 rank of Frontal Deflector; the final upgrade is either the fourth health or the debris vortex). Since I'm not at war with the person I'm building around, there's no risk to the constructor, no need for a fleet to guard it for two minutes. This policy has several practical effects:
1> Enemy ships can't easily attack me by passing safely through my erstwhile "ally", who also has a cease-fire with my enemy.
2> I keep vision on their planets regardless of whether they agree to a vision pact or not.
3> I can phase-jump through their territory to hit distant enemies.
and most importantly,
4> When the Cease Fire inevitably breaks, I've got a series of massive combat bases parked right in the center of their infrastructure, with phase gates allowing my fleets to jump straight to the systems of their choice. The starbases will proceed to fight the enemy starbase (if any), followed by clearing out the enemy's defense structures, and finally any commercial ones.
I send in my fleets to help, but take last night's game for instance; I did this to 12 systems on one opponent before the fighting started (and he did one to me, in return, placing a starbase around one of my key planets). In three of those twelve cases, the enemy would have destroyed the starbase had my fleets not arrived (and in one case, they managed to destroy it even with my fleet there); these three were the systems in which the AI had placed his own fully-upgraded starbases as well. Most of my fleets were busy wiping that foe's battle fleet, which he'd been using to attack a third party at the time.
But in the other NINE cases, my starbase won the battle without any fleet help at all; some of those were where the enemy starbase was outgunned (and massed Kostura Cannon assistance really helped), but most were systems where he'd never built an SB, period. Hangars and gauss platforms are just no match for a slightly mobile base with 20k hit points and huge guns. I'll say that again, in most cases the starbase WON. By the time my fleet arrived at some of these planets, every orbital structure was destroyed and no ships survived. That's a huge strategic advantage, not just in time saved, but in the risk to my own planets; half of the enemy's ships were pinned down in scattered losing battles against my bases, instead of being sent towards my own systems to raid. One of the systems cleared this way was a chokepoint between his home system and the rest of his empire; that base effectively blocked any reinforcements from helping his other systems, and made conquering easy. I still had to send fleets in to perform the planetary bombardment, though.
So, in that sort of strategy, an anti-planet starbase weapon would have tremendous utility. Put it on a starbase near one of your allies, and if the alliance ever fails, the starbase won't just wipe out the orbital structures, it'll wipe the entire planet. Send in a colony ship and you're done.