Most people who use twitter regularly have encountered this situation in their twitter feed. You see a tweet from someone in your feed, and then you see someone else retweet it using the old RT @username style. I find this to be incredibly frustrating, since it's wasteful of my time. I am here so say, right now, "Stop all the RT'ing!"
First Reason: Redundancy
As I laid out above, when people use the RT, they run the risk of creating duplicate entries in the feeds of your followers. This becomes especially bad if multiple people in your feed RT the same thing.
Second Reason: Powerlessness
At one point, not too long ago, many of the people that I follow were Retweeting (referring to the official, built-in retweet style) @horse_ebooks. I found these funny at first but grew to become annoyed by them. To fix this, I just blocked @horse_ebooks and it stopped showing up in my feed.
Using the Retweet functionality and not the RT allows your followers to remove/block those tweets from their feed. You can not only block Retweets of a single source, but you can block every Retweet that a single person makes by clicking a box on their page. When you RT, they have no way of doing that.
Major Faux-Pas: The double RT.
The double retweet
is when you RT something that someone else RT'd. "RT @dudebro1 RT
@dudebro2 Look at this wound I got while skateboarding!" is a good
example. I will murder you if you do this, seriously. The purpose of the
"RT @" notation is to credit the original tweeter. You don't need to credit the person who brought it to your attention.
Last Reason: Consequences
I have unfollowed people in the past and recently because of their RT'ing. This person would retweet often, and usually from the same source. I eventually got tired of it and, having no way to filter them out, had to unfollow them. I didn't like having to do that, but I did.
I only see two good reasons to use the RT-style retweet. The first reason is to answer a question publicly, so that you can include the question with the answer, such as "Super Mario Bros. RT @gamedude007 What's the first video game you played?"
The other reason is if you are a celebrity and want to lend the weight of your celebrity to a cause, such as "Everyone should check this out! RT @charity We just launched our new site for that thing that needs help (link)".