Sorry, Pop, you don't have an intuitive feel for the amount of CO2 generated by the power, cement and transportation industries. You'd probably have to slime every square inch of earth's land are with algae to accomodate all the CO2 generated by consumers and industry. Granted CO2 is a useful input to some industrial processes, but there is an enormous imbalance between CO2 demand and CO2 "supply".
Well, the DOE says the US produces about 7B tons of CO2 per year. Of that, as a wild guess, let's say 2B tons could be put into biodiesel production, though that is probably a liberal guess.
Algae consumption of CO2 gas is about 60% efficient, so perhaps it is not as unreasonable as all that.
Every process that we use only makes sense at the low end of the curve, before the law of diminishing returns kicks in.
But it still makes of lot of sense compared to throwing CO2 away.