Well....it has to be "harvested" and grown to be used (maybe we're not understanding each other). A big advantage of using switchgrass is that it grows even in very poor soil conditions, grows in diverse climates, requires low levels of fertilization, and is perennial (I think this is what you were referring to). Also, switchgrass can provide as much as 10 tons of biomass output per acre.
The big challenge facing switchgrass is its energy efficiency. I'll have to look this up in a bit, but I remember there was a report done by a prof at a school in California (might have been Berkeley?) that demonstrated that it took more energy to produce fuel from switchgrass than the amount of energy switchgrass produced. I think this is still open to debate, though.