Malate shuttle vs. glycerol phosphate shuttle

[nitroglycol]

I know that these serve to transfer electrons from cytosolic NADH to the respiratory chain. I also know that the malate shuttle generates NADH in the mitochondrion, while the glycerol phosphate shuttle generates FADH2, meaning that cells that use the malate shuttle will get more ATP from a molecule of glucose than those that use the glycerol phosphate shuttle. What nobody's been able to explain to me, though, is why any cells use the glycerol phosphate shuttle at all. (And yes, I know that different cells in the same organism use different shuttles). Is it a problem with enzyme regulation perhaps?

Edited January 29, 2009 by nitroglycol