I often experiment around with how a track would sound if played with other instruments. However drag-n-dropping a new instrument or preset into a track is not undoable yet. Sometimes it's especially annoying when you place an instrument by accident into the track beneath or above the track you initially intended to drop the instrument on. In this case i often dont remember what instrument was in the other track to fix my mistake.
Being able to undo instrument changes would be very convenient.