I'll bet they had already declared minimum fuel with approach and maybe even asked for 31L and been denied. The crew probably came to the conclusion that if they got one wind report out of tolerance they were declaring an emergency and landing the plane. If they hadn't declared an emergency, they probably would have been vectored all over creation while the controllers attempted to accommodate their request for 31L. I don't think they would carelessly pull the emergency card unless they absolutely needed to.