Thom has an important role early (I brought it up on Twitter).  He legitimizes the mistrust of the Aes Sedai to the viewer.  Without him you only have the hick or the extremist viewpoints on the Aes Sedai, neither of which would be trusted by modern audiences.  Thom on the other hand is well travelled and obviously intelligent, much more trusted source.  Even the tragic story of his nephew gives legitimacy to the thought that Aes Sedai aren't necessarily all good and nice.  Without him the mysterious and potentially dangerous Moiraine becomes a gender-bent Gandalf (even more so than before).  While Thom's accomplishments later in the series could be overlooked, he helps adds to the drama and tension early in the series.