Is local equilibrium a reasonable assumption for evolutionary processes?
Nothing is at a genome-wide local equilibrium. Graham Bell wrote fairly extensively on this (IIRC). 

Some loci


will be at what are likely global optimums (e.g. Cytochrome oxidase)
will be at local but not global optimums (e.g. low-fitness malaria resistance vs. high fitness malaria resistance: for the extremely cool story check out this page)  
will not be at any optima because the environment shifts too quickly (e.g. loci clearly under frequency-dependent selection, such as MHC). 


So, in general, it depends what you are trying to model. If you want to model a specific kind of evolutionary change, then model that kind of evolutionary change. If you want to model a different kind of change, then model that kind of evolutionary change. Natural populations will likely follow any model some of the time, but will probably never follow any model all of the time.