Tweetbot lets you chose how long to mute for, a day, a week, a month, or forever. Handy when a company you follow starts doing a contest and they're retweeting the best entries, or someone starts a Q&A you're not interested in, etc.
This is why I'd rather use the mute button over unfollowing someone. For example, I use Tweetbot and I follow Expedia, but when they do their Twitter Q&A, I generally mute them for a day, since I don't want their tweets and RT's cluttering up my timeline. So in that scenario, muting someone is much better than just unfollowing them.