Twitter is a social networking site.
How are you going to be social if you are auto tweeting?
How are you going to know when your target audience is around if you are never there to see the stream?
It is also annoying when you try and be social with someone that auto tweets and never replies.
Auto tweeting might be ok as a filler on off hours but you still have to take an interest in your followers if you want to build a community.