You have to wait for it to build naturally anyway. If it is at the front of the queue, all your income would be funneled into whatever is at the front of the queue. So if your income rate is greater than your build rate then it will still build just as fast.
No, because you would not cancel building whatever is building already, and what you need built immediately would simply be next in line. So, resources would still be spent building what was already in the pipe, and you would have to wait to gather enough to start the next build. If you needed it immediately, you would have to cancel what was already in the pipe, both for time and resources.
Perhaps you could elaborate on this a little? Any resource gathering in a REAL time game has a resource income RATE. In SupCom you can increase that rate infinitely where as in Sins you can't (we discussed this in another thread) but both SupCom and Sins have a specific resource income rate at any particular point in the game. What are the issues with funneling that income rate into the item being built at the front of the queue? I am sure there are technical ones but I don't see anything but positives in terms of game play mechanics and freeing up the player to plan his/her build strategy as far (or near) into the future as he/she wishes.
While Sins has resource income rates, building/researching still deals in absolutes. You can have an income of 100 crystal/minute, but you still can't research that tech unless you have x amount of crystal. What I was saying is that in order to have a system where you can continuously queue and where resources are spent as the building progresses, the best resource gain system to implement would be like the one in Supreme Commander, where absolutes don't exist. You don't need x metal or x energy to build something, you just need to balance income rate and expenditure rate.
Let's toss another game in the mix: Homeworld. There, you had absolutes in resources where everything cost x amount, but production spent resources as it went along. So, if you didn't have enough you could still start building, it would just halt eventually until you got some more. This led to complete halts in production, and if you needed something built or researched quickly, you had to cancel everything already mid-production so that you would have resources to build. In Supreme Commander, nothing halts completely. If your expenditure rate is greater than your income, at worst it slows down, but you can still build.
Does that make better sense?
Granted, in Sins you don't need to wait for harvesters to return to your HQ or whatever to get your resources (in the beginning), but the rate of gain is so negligible to expenditure rate that even though you gain metal/crystal continuously, for all practical purposes you might as well have a complete stall. In late game, when all the asteroids are depleted, you *do* actually have to wait for refinery ships to dock for your metal and crystal, and then the problem becomes pretty much exactly like Homeworld. Your whole production line stalls, and if you need something immediate, you have to cancel what was in the middle of production to recoup enough resources.
In Suppreme Commander, if you really need something fast, at worst you pause construction of other things. You don't have to cancel. Just stop your engineer from spending on one structure, decrease the expenditure rate so that you're not slowing production of the other thing, and then go back to finishing. You don't have to cancel anything to recoup any x number of resources.
Hopefully this clarifies it

EDIT: Furthermore, we don't even know exactly how the other 2 races gain their resources. The Vasari are supposed to use planet suckers for resource gathering, who knows if they even build asteroid mines for a constant rate of increase, like the TEC.