Have you ever seen Brandon Tate play? I have.. He's pretty damn good. And, prior to Moss, who were these "down field threats" that the Pats had. Branch wasn't one. Neither was Givens. Yet, the Pats played extremely well.
People like you seem to have forgotten the lack of quality at WR from 2001 through 2006. The Pats, at best, had 4 "#2/#3" type receivers. But that is stretching it since Brown was on the downside of his career in 2003 and Patten got benched before the end of the season in 2004.
Could the Pats add another WR? Well, sure they could. But much of it depends on where they finish in the play-offs. IF they finish in the top 4, then they won't be allowed to sign anyone unless they have lost someone of equal value. And there are restrictions if they finish in the top 8 as well.