Lately, I've been playing a lot of the big multi-star maps and I've found I am relying a lot more on culture.
As a defense, I'ev got to say, it works awesome. It might not save your planets but it does keep the enemy from getting a beach-head. They may be able to kill your defense forces before your fleet arrives, but it won't do them any good if they can't colonize the planet. Plus, you get the bonus for fighting in your own culture. Late game, one or two planets doesn't really matter on a huge map, as long as they can't be used against you as bases.
But, I've found it works great as a offensive weapon as well. I've been making sure to drop at least one broadcast center on each new world. Along with culture tech's, I can have at least one or two worlds in front of my fleets under my own culture. This gives me great advantages. First, I can Novalith the enemy worlds directly in front of me and they can't be re-colonized. Second, when I do attack, I get inculture bonuses. Plus, if I 'buy' some pirate raids, the enemies have a lot tougher time re-covering, and if they loose the planet, it's gone.
I kind of think of it as a 'planet-lock'. With enough novaliths and broadcast centers, you almost don't need a fleet. Buy some pirates, have them clean out the fleets, nuke the planets, and then just colonize. I've even been leap frogging planets since I don't have to worry about the un-colonized ones behind me getting infested.