I wouldn't agree that a tweet without a reply or a retweet is ignored. If somebody reads a tweet, and just for an instant thinks "oh, i agree", or "what an idiot", the tweet had some impact. Of course, it all depends on the way you use and look at twitter. There are many "lurkers" out there that just like to follow interesting people and read their thoughts and opinions, and don't feel the need to reply or retweet every single tweet they come across.
On the other hand, a lot of them should be ignored. I really get a lot of tweets that are part of conversations or some personal stuff. It is a shame when a smart person that sometimes has really interesting tweets, "spams" you with unimportant tweets talking with his friend he just saw 10 minutes ago and will see tomorrow first thing in the morning. Common, use mail, IM or facebook for that. Sometimes I feel like I am intruding on a phone call. I know, who am I to tell them how to use twitter. Of course, I don't want to. But, that is when people stop following them and their tweets get really ignored.
On the other hand, a lot of them should be ignored. I really get a lot of tweets that are part of conversations or some personal stuff. It is a shame when a smart person that sometimes has really interesting tweets, "spams" you with unimportant tweets talking with his friend he just saw 10 minutes ago and will see tomorrow first thing in the morning. Common, use mail, IM or facebook for that. Sometimes I feel like I am intruding on a phone call. I know, who am I to tell them how to use twitter. Of course, I don't want to. But, that is when people stop following them and their tweets get really ignored.