Right. I'm not seeing how that's especially relevant. There are a bunch of complaints about total cost, but by the time that becomes real factor, then you should have enough production that the 2 extra alloys are worth it if you want any real pop growth at all - instead of having .3 pop growth for free, you'll get 2.3 pop growth for 2 alloys from the planet. The only real consideration is the opportunity cost in terms of pops.
At some point, that cost will justify phasing out the job, but on full, small planets, getting say, 10 extra growth points when you need 500 from planets that are generating 1.5 means you're getting more than 5 times the total growth. For 10 alloys from an economy that should be generating several hundred, 10 alloys is worth it when it's your only real pop growth.
Now, if you're playing a heavily aggressive style, or raiding other empires for pops (which is now very, very attractive), then it's probably not worth it, but neither is the normal organic pop growth either. Holistically, if you're trying to extend your empire's growth for as long as possible, conquering the galaxy isn't even worth it - you want several small, defenseless, inevitably angry empires that you regularly declare otherwise meaningless wars just to steal their people.
1) You really shouldn't allow your colonies to get full. It's counter-productive if there are empty planets anywhere. Just arrange your job count to be half your capacity.
2) That means you are generally looking a 6 pts of growth for biologicals. So the 2 pts of separate growth on assembly is incidental after a while. Before I quit, my longest game of 3.0 had my biologicals take about 6 years to grow and the robots about 18 -- assuming the growth penalty didn't get worse in the next 18 years, which it would because that is 2.5 cycles of biological growth. It also means each robot was consuming an opportunity cost of 2 destroyers over 18 years -- not considering the hefty energy cost as well. That's 80 destroyers over the 40 or so colonies I was running, which was larger than my fleet at the time.