What you see depends on where you are and where you look.

The enzyme which fixes carbon into phosophglycerate, rubisco, is very ancient and rather easily confused — left to itself it will sometimes grab oxygen molecules rather than carbon dioxide molecules, and instead of making phosphoglycerate makes phosphoglycolate. This is no good to man nor beast nor, most tellingly, plant: recycling the phosphoglycolate made accidentally in this process of photorespiration into a form of carbon that can be used for further photosynthesis takes energy, and thus making less phosphoglycolate in the first place is a good thing. The malate-initiated photosynthesis that Hartt was instrumental in discovering is an evolutionary response to that problem: malate is part of a clever biochemical/physiological supercharger that concentrates a great deal more carbon dioxide into the cells where rubisco is doing its thing, thus making it less likely to commit that costly error with the oxygen.This supercharging system is known as C4 photosynthesis, the 4 denoting the number of carbons in malate; the regular sort of photsynthesis is called C3 in contrast.

C4 photsynthesis confers various advantages: in particular, it makes plants more efficient in their use of water. The mechanisms that concentrate carbon dioxide mean that the pores through which it is taken up, the plant’s stomata, don’t have to be as wide open as they would be otherwise, and thus less water is lost. C4 plants resist various sorts of stress better, including direct sunlight and salty ground. The mechanism has evolved independently many, many times over the past 30 million years or so, mostly but not entirely in the grasses, which either have a propensity for the sorts of physiological re-design that is required or are particularly prone to finding themselves in the sort of niches where this approach helps, or both. Sugar cane is not the only domesticated or agriculturally relevant example — there’s also maize and sorghum, and for energy crops switch grass and miscanthus, among others. There is now considerable interest in building the pathway into some grasses that have not learned it naturally — most importantly rice. C4 rice, with higher water use efficiency and other extra hardiness, might have considerably higher yields than traditional varieties while needing less water . . .

This is true, but not the whole story. C4 plants are noted for being less nutritious. They often have more sugars but they have less protein and minerals. The higher protein content of C3 grasses is due to the abundance of rubisco (it is said, I can't prove this is true). They are also more palatable and more digestible (same caveats). I'm not sure if this is necessarily so. For example, C4 grasses that are grazed are poor forage from the perspective of the grazing animals. They are often more productive in the dog days of summer, but not in spring, fall or winter. And, they have less balanced nutrition so animal health declines.

People only eat grass seeds so they don't much care if leaves and stems are less nutritious. The same is true when these plants are used for fuel. It's the carbohydrates that matter most. What's more, people use these plants in "batch mode" rather than as a continuous process. The plants are left until they get rank and go to seed, long past their more nutritious vegetative state. But when used for forage it's the opposite. The plants are harvested repeatedly, with a rest between harvests. They don't get rank or go to seed until very late in their life spans, just before they die or go dormant.

It is possible to develop cultivars that perform better during their normally slow times, but still have good performance in their preferred season. I grow C3 perennial rye grass that never goes dormant. I hunted the world for a variety that met my requirements and chose one developed for New Zealand by a Dutch company. It is highly productive in spring and fall, more productive than C4 bermuda grass in summer, and even grows a little during the winter when the photoperiod is brief. It beats C4 grasses every month of the year even though production is reduced in the hot (very hot) days of summer. I have a nice climate and sufficient water, and pamper my pastures so that they have good soil organic matter yada yada. Other land in other climates can't do this so well, or perhaps at all.

What this suggests to me is that we might get more benefit from seeking to develop plants that yield multiple crops. Using the rice example, more food would result from multiple harvests of C3 rice than from a single harvest of a notional C4 rice. Faster growth in early season and earlier maturity could even achieve the water use objectives since water is less scarce at those times and less is lost to evaporation than during the hottest part of the year. Best of all, do both. There are bound to be situations when one or the other works better. A whole spectrum of cultivars is the ideal, with seed suited to every location and agronomic system. Mega-diversity.

When we contemplate improving plants it can be useful to consider the big picture rather than tightly focusing on a single attribute and trying to optimize it. That's much harder to do, and for pragmatic reasons we may well do it a step at a time, but keeping a larger long term goal in mind can perhaps avoid overshoot and dead ends.