I was actually talking about refineries, not extractors.
You said resource extractors, hence I remarked about extractors 
Yes, refineries on low loyalty planets are usually inferior to trade ports. However, the extractors remain quite lucrative.
Not my point, i was saying that the neglible extractor income from a planet with very low allegiance is not worth it because colonizing that planet requires:
- A fleet presence to protect it during development
- Time that could be spent attacking the enemy
- Attention, you will be focused on that planet allowing your enemy to posibly outmanuever you
For your first point, unless you've got a miracle choke point, an enemy fleet is going to break any static defense. The only real defense is a good fleet on standby. If the enemy has the power to retake the world by brute force, you may be more inclined to start building up repair bays there to absorb his eventual counter-attack, and if he doesn't you can always return back to chase off his fleet.
Secondly: who said you cannot attack the enemy while you hold the new planet? If he's busy defending himself, he's hardly in a position where he can send a dedicated assault force. Even in the case of an AI which loves siege frigates, you can leave behind a single carrier or even keep a roaming group of scouts (scouts are really good at killing siege frigates).
I don't see your point with attention. Unlike games like Supreme Commander where you constantly have to manage workers, queue upgrades, and balance resource incomes vs expenditures, everything in the Sins economy is a one-time purchase. You buy it and forget about it. The attention required doesn't even begin to match up to something like a two-front war; if you're getting seriously distracted by half a dozen one-time purchases, you're going to have a lot of very serious issues that dwarf this one.
but it hardly seems worth the time since culture overthrows take forever.
Culture overthrows aren't the point; in friendly culture your units get a bonus. Replacing enemy culture with your own is two birds with one stone: you take away the enemy culture bonus, and gain your own!
I believe you get the same tax bonus, no matter what you pick.
No, desert and terran planets give the highest bonus, ice and volcanics give a medium bonus, and asteroid homes give a small bonus. Pick a terran or desert when at all possible. That said, loyalty impact on resource extractors is going to overshadow this!