I like to evolve ideas when recording. Seeing as I don't pay for recording time (can do unlimited takes) and don't want to just duplicate a practised part to get just the best version of it, it really helps me to hear a previous take to build new ideas from. This comes to the forefront when creating dual lead solos or parts that harmonize with each other.
 
The work around of copying to a second track works but is cumbersome and wasn't necessary before comp recording.
 
If this was added to the new Sonar model I would jump at it!