It all really depends on your application, all the lenses I use on a regular daily basis are considered to be classified as fast for their respective focal lengths, examples; SMC Pentax-A 50mm F1.2, SMC Pentax-A* 85mm F1.4, SMC Pentax-A* 135mm F1.8, SMC Pentax-A* 200mm F2.8 ED. It all depends on how and what you want and need to do, in any given situation.
As always, most lenses perform better when stopped down a stop or two, but there are real world situations that will not allow this and sometimes shooting wide open is the only route. For me that's music concert photography, where there is no flash allowed and ISO is already cranked to the max.
Remember the viewfinder always looks brighter with fast lenses and for manual focusing in low light situations, trust me, that's needed due to the much reduced DOF, which can be wafer thin is some cases.
But hey, that's just my world.