That has been suggested time and time again, however, w/ FNA and FoE merging operations, vehicle lines, etc. in a few years/next model generations, causing almost identical FNA and FoE vehicles, Mercury would then, in a short matter of time, become another Ford rebadge.
Besides, at the moment, according to an article posted somewhere on here, some FoE products (S-Max, other Genk(?) plant models) have a 6 month waiting list because the plant cant make enough of them. So, it sounds like FoE doesn't have much excess capacity to ship models over for US consumption as Mercuries.