Because it optimizes screen real-estate, and is really useful! Many users are very disappointed that you have removed this feature, even though it was experimental. Perhaps we are only the minority because the feature was never publicized.
If it's really not appropriate for the core application, then please extend your plugin architecture to accommodate this modification.
Nathan Broadbent started this petition with a single signature, and now has 214 supporters. Start a petition today to change something you care about.