When one topic breaks out of the pack, there's something more important going on. When a large mass of people have all turned their focus to a "trending topic," Twitter needs to treat that differently than the ordinary "tweets" of individual users. Little changes in the Twitter service can have a huge impact on everybody, since it will literally effect hundreds of thousands of Twitter accounts. And if something has suddenly emerged as a "trending topic", there's a compelling public interest in making sure it gets all of the attention it deserves.
In some cases, the urgency is obvious. The "trending topic" could be a hurricane, a sudden flood, a terrorist attack, or some other catastrophe which needs as much attention as possible. But now let's assume that on the same day, there's also several juicy pieces of celebrity news which are rising up the "Trending Topics" chart. Is it really in the public's best interest if those important safety updates get pushed out because there's slightly fewer users commenting on them?
And this will become more of a problem as Twitter continues to grow. Since Twitter has millions of users, any given day will find somebody posting on nearly every topic. But soon there will be huge numbers of people, swelling the number of updates for even the most trivial topics. Andy Warhol once predicted that everyone will be famous for 15 minutes in the future. Unfortunately, that's the very thing that could destroy the usefulness of "Trending Topics."
Twitter is simple, but that doesn't mean it isn't powerful. And I really do believe that there's some important social need that's served by Twitter's "Trending Topics" list. Individually I'll never be able to determine what's the most important topic at any given second of the day. But Twitter is a classic example of the "wisdom of the crowds," where a large group of people can, collectively, sort things out better than any individual. Since I believe that, I also have to believe that there's something special about the updates which can achieve critical mass. And at that point, I think Twitter needs to preserve that collective wisdom, so it's around to benefit more Twitter users.
Another problem with the "Trending Topics" is it's deeply affected by time zones. If you live in Hawaii, most of your trending topics in the morning are determined by the people who live west of you - in non-U.S. countries like Japan and India. And towards the end of the day, it's already much later in the evening for most of the people in the U.S., so you'll run into the same problem. This is a problem that's experienced today, and it's directly related to the simple programming in the current Twitter interface. And it's one of the first things that would be improved if Twitter began doing more to save the "Trending Topics".