If it's used as a tool to help one's life, I see little wrong in it. I'm a thorough rationalist, and I use it as thus.
But then it's because most see it as the newspaper horoscopes, which are shit. It's a far more complex field than one would note at first.
And the characteristics of each Sun sign are only meant to be shallowly applied....no two Leos or Aquarius's are the same. If anything, Sun sign is probably the least accurate sign to use, Moon and Ascendant/Rising are most accurate.