Apparently they have the best fleet DPS, speaks for itself.
Actually, Advent has that distinction. If you complete the phase missile upgrade line, that's another matter entirely, hence why Vasari is most feared late game.
Why bother using defences when you can build Phase Stabilisers?
Defenses can be useful, but if a big fleet slams into them, they're just going to get torched. Fleet is the only counter to fleet, period. Defenses exist to support fleet, they are not stand-alone.
Why bother building ships when you can build Phase Stabilisers?
Obviously because you need ships to actually travel through the phase stabilizers to defend.
Why bother building starbases when you can build Phase Stabilisers?
Starbases don't count to command limits, and are vastly superior to your usual turrets and hangers. When you have a repair bay around, they can ward off some pretty substantial enemy assault forces.