It would be awesome to be able to extend service items and use comments. You can currently do this on a service only sale but not simple or advance sales.
Example would be a company who also includes labour lines on their orders, but want to show some details about what the labour was for, without using notes on the whole order.
Attached example of what you can do on a service only sale, but not others.
You can close this as I voted on the existing feature request.