in general, the sweet spot of a lens in two stops down, so the sweet spot of the a f2.8 lenses is at f5.6, and a f4.5 lenses is about f8.
beside, most f2.8 lenses are sharp enough when shooting wide open, but the most important things is the f2.8 lenses have better contrast and color reproduction compare to those f4.5 lenses.
another benefit of using a f2.8 lenses is you have a brighter view in your viewfinder.