As many benefits as there are to the use of biomass for energy, there are also plenty of reasons to remain wary. Most of the downsides have to do with the production of the biofuels.
There is a finite amount of land suitable for agriculture on the planet; using more of it to grow crops for biorefining necessarily means that there will be less space for crowing food for hungry populations.
Growing biofuel sources, whether corn or soybeans, works best as a monoculture -- endless fields of the same exact crop. Monocultures are extremely vulnerable to disease, since each plant is a clone of its neighbor, they all have the same weaknesses. A bad pathogen or a nasty stretch of weather could take leave a great many consumers without electricity.
Furthermore, only large-scale operations would be able to produce enough biomass to be profitable, so local farmers would be cut out of the equation.