With M4L you need to implement your feature in a device and add it to your project. My Ableton project template has a bunch of these on my main track. With extensions you use a context menu as the entry point which will hopefully be more lightweight. Hopefully they'll expose more of the object model over time and let us trigger these via keyboard/midi shortcuts.
When you are transcribing a guitar part you have to think about the key, the chords, and how to play the part efficiently. It's a great way to get a better understanding of the neck since there is usually more than one way to play a part.
