Up to a very small point, consider:
1) There aren't enough plants to absorb excess CO2 at the rate required to match the current (or future) excess emissions. 2) As mentioned, plants need CO2 to live, but give them too much and the vital nutrients they produce, become depleted. These include iron, zinc, and vitamin C. 3) Overall, FACE experiments show decreases in whole plant water use of 5-20% under elevated CO2. This in turn can have consequences for the hydrological cycle of entire ecosystems, with soil moisture levels and runoff both increasing under elevated CO2 (Leakey et al. 2009). [ed. increasing the potential for flooding, landslides, et alia] 4) Crop concentrations of nutritionally important minerals including calcium, magnesium and phosphorus may also be decreased under elevated CO2 (Loladze 2002; Taub & Wang 2008) 5) With elevated CO2, protein concentrations in grains of wheat, rice and barley, and in potato tubers decreased by 10 to 15 percent in one study.