I don't think anyone takes the NFG rankings THAT seriously. It's nearly impossible to rank fighting game players' actual skills, or even their tournament accomplishments. But the NFG Rankings are an objective fun system that provide fuel for some debate and can make players strive to get better results even more than they already do (think @KTSmith's excitement over making the rankings).
And if a newcomer wants to know who the top MK players are to watch out for (say a non FG person watching MK at MLG), they're a nice shorthand.