Yes, Ardilus, I am yet to get there, the middle game, but did it make any real difference for economy?
Undoubtedly. There is the obvious direct effect of paying more for assets, though most price fluctuations are admittedly minor. On rare occasions, however, you can see some serious inflation. If carbon fiber triples in my current game, for example (and I've seen worse), my cost to build military ships goes up about 15%. There are less obvious, indirect effects as well. Your private economy will suffer not only paying higher prices if you do not have sources, but cost increases mean galactic shortages -- those long construction queues not getting resolved for months or years also include your freighters trying to upgrade. If you take half your freighters out of commission for a year, you will feel in all kinds of nasty ways, and for a long time.
I guess my point is - wouldn't it be better if resources were either finite (like in space empires) or just less of it gained (e.g. through gas/mining stations),
that way I could actually care what planet to colonize or conquer for its precious resources. You know, like in a real war.
Actually resources are finite. You can see this on your colonies, for example, by looking at the <resources> tab on the colonies screen.
I often target colonies for rare strategic resources like chromium. But, of course, in a real war sometimes you just need to occupy Midway Island for it's location.
You will want to keep in mind as well that as your tech increases, required construction resources change. As a gratuitous example, in my current game all my cruisers are equipped with super death rays, and as such require more iridium and dillithium each than steel! If I do not have lots of dillithium sources, which are pretty uncommon, I can run into real trouble.
With all that said, DeadlyShoe sums up very well the trouble with resources on the local level, which is usually the big concern.
Now, year 30 in my campaign I have colonized about four planets and private sector expending nicely, and resources mean little besides few special ones. (playing normal/expensive research). I see little point to target planet for its resource.
Forgive my saying so, but only 4 colonies in 30 years is without question a losing game. I don't see your empire able to support the force sizes that would make either local or global shortages much of an issue, but it probably won't matter when the Sluken find you