Once watching own replay I figured own very obvious and very crusial thing.
In this replay I had around 35-40 harvesters on main+natural and income around 900-1000 minerals (per minute?) When I detected lack of harvesters I called 4 (or 5) MULEs. After that (if we believe to replay statistic) my income was JUMPED TILL 1800-1900 per minutes. It was kept on this level for 1.5 minutes...
It's obvious but it's so huge deal. That means that in 1.5 minute I get (almost for free) 1k5 minerals (30 marines!!!)
Plus add this topic StarCraft2: MULE - can he work on one mineral patch with another SCV in parallel?
Here I can conclude that often when: - you plan to expand but due to some reason can't move your CC to natural; - OR you plan to have 2nd expand but need to clean-up enemy forces from new location; - OR don't plan to expand but temporary have exceed of minerals
it could be a good idea to create a new CC on you main build a OC just to call for MULEs. Also, id you DO have expand or 2nd expand... It seems like very useful to build ONE MORE CC+OC just for CALL mules...
Usually Terran plays main+natural, and having 4 CC doesn't seem to bad for me. 4 CC means that 1 will be used for scanning and 3 more will be able to provide MULEs for 2 bases...
I understand the drawback of such strategy: instead of building production building and army you will increase economy strength and it is necessary to balance... But having enough minerals temporary you could put them in a CC+OC - and will pay for himself very quickly.
Am I right? Is there something that I missed here?
Edit: Forget to mention - CC is an additional supply, so it's actual cost is not 400, but 290 only! Though OC costs additional 150 minerals