True, but in the first part of the game, your capship's ability to repel culture is still relatively low level and you can't really afford to have it sitting doing nothing in a system.
If you have 35% allegience on a planet, you only gain 35% of it's income. If you can raise that by 5% with culture you've just improved the value of that planet by 1/7!
Perhaps a better way of looking at it is the 10% bonus to all planetary resources that are in your culture.
Here's an Advent analysis:
Cost to get your first Temple of Communion: two civic research centres (750/60/80) + building research (600/50/100) + building cost (900/100/150) = 3000/270/410
...or a little more expensive than a capital ship. Note that this also gives you Tier 2 Civic research, so it comes with a bonus.
Now, this will boost the output rate of your Temple of Communion's planet by 10%, and then the planets closest to it by 10% after a while. In most maps there are 3-4 colonizable worlds that are phase-lane attached to your homeworld, but let's do the worst case and say the only thing nearby is an asteroid with at minimum 1 metal, 1 crystal.
So you're getting a 10% bonus on the tax rate for 300 population after full infrastructure upgrades, plus 3 metal and 2 crystal asteroids, or an overall benefit of about 1.5/0.1/0.2 per second until your culture spreads to further planets. In the worst case
, it doesn't really pay to build a Temple of Communion at your home world just for the economy boost
because it'll take about 2000 seconds, or 40 minutes, to earn the complete cost back. This doesn't factor in the benefits when it reaches a third planet, or the time it takes to change your planet's allegiance, but it's a pretty serious economy hit for that critical first half-hour of gameplay.
Moral of the story: do not build a Temple of Communion at your home planet. Building it the second or third node out will provide a better bonus.