1st) if you don't interfere by any reasons (you don't have time, you don't want to, you can't deal with a planet full of primitives) Observation post is basically another research station. (also don't forget that i'm talking about pre-atomic(or even pre-steam primitives. cuz they are usually not worth the enlightenment in my eyes)
Well, lets go over your reasons. "Time," it's an automatic process that costs the click of a button, their time as protectorates is similarly automatic, as is their conversion to a vassal. "Not wanting too" I'm not even going to address except to say if you don't want to play well, I can't help you but I'm also not going to take your opinion seriously when discussing game balance. "Not wanting to deal with a planet full of primitives" you understand that technological enlightenment stops them from being primitives right?
You actually missed the biggest reason not to, which is initial resource investment. You sorta cover it later so I'll respond then.
(also don't forget that i'm talking about pre-atomic(or even pre-steam primitives. cuz they are usually not worth the enlightenment in my eyes)
Okay, you're still going to be wrong if the game runs long enough to get a return on that investment, so all this tells me is that you either only play medium sized maps or start technological enlightenment to late. The overall value of 5 energy and 11 society research a month drops significantly as the game advances. Regardless of what method you use to determine when, any primitive tech level will reach the point where it's a worthwhile investment to enlighten them, even if they're in the bronze age and you're only doing it because you've reached the point in the game where you simply don't notice the cost.
2)yes. you'll gain 0.5 influence from protectorate...but later you'll need to spend 5 influence per month to integrate them.
That's true. But you're making your judgement off of prospective costs and not spent costs, you can't do that because you're not looking at the total return just the return from the protectorate. If you're ONLY looking at .5 influence a month, then you have to ONLY look at the cost of getting it. For a pre-steam primitive that's going to be at least 1000 energy and 2200 society research when compared to using the same observation station for active observation. Not worth while early game, but mid game when energy and society research are cheaper the balance tips. Late game it's not even a comparison, especially for large empires, and the question stops being resource investment and becomes one of "will I still be playing this in 200 months?" and "Will it last long enough after 200 months for me to get a return?"
If you want to talk about whether it's worthwhile to incorporate a vassal, lets say one on an 18 population world. It becomes a question of whether it's worthwhile to spend 200 influence over the course of 40 months to get a fully developed 18 population world with a space port. Given that I've spent 200 influence to forward settle a 1 population world with no development, that's actually a much easier question.
If you're looking at technological ascension strictly as a means of obtaining a new world. It's going to become 50 minerals, 1000 energy, 2200 society, and assuming your mid or late game protectorate takes 5 years to become a vassal, that's 170 influence over the space of about 400 months at the end of which you get a fully developed 18 population world with a space port.
3)if you want fleet power - get a planet and build spaceport6. it'll be cheaper and faster cuz vassal's fleet is usually non-existent.
I am, doing one barely interferes with the other and so it's easy to do both simultaneously. The resources spent to get a planet and build a spaceport6 are significant minerals and a small amount of energy and influence. The resources spent to technologically enlighten a pre-ftl civilization and then have them become a vassal are time, energy, society research, and a small amount of minerals. The overlap in resources is minimal.
and for conquering: 5*60 minerals for assault armies vs 350 for colony. pretty much the same cost.
You can conquer an FTL civilization with 3. So it's 180 versus 350, or nearly double. You can also conquer a second FTL civilization with the same 3, so it becomes 180 versus 700. Or nearly a quadruple.