Retweet is a button that makes forwarding a particularly interesting tweet to all your followers very easy.The plan is to see how it goes first with this small release. If it needs more work, then we'll know right away. If things look good, we'll proceed with releasing the feature in stages eventually arriving at 100%.
Since the "official" RT button doesn't allow appending one's own comment, very few of the users who have been given access to it like the feature.
Twitter has done a lot of good things, but I don't think they have fully grasped that they are a community and that changes need to be explained, discussed, and tweaked to the needs and desires of the members. You'll never please everyone, but if you listen and adjust you can please some of the people and at least make the others feel like they've been heard.