I understand the need to be able to see how the horse is going before jumping aboard. When you are already in the saddle, it's WAY too late to be finding out that the horse's education is lacking, he's fresh as a daisy or can buck like a bronco.
The problem with using lunging to check this is that you have little control over whether or not he bucks, feels chased or exhausts himself in the process.
If we remember that we are always teaching patterns then it's easy to see why lunging, especially at speed with little control, is a dangerous habit to build. I don't think it makes any difference to the horse whether you are riding or not - if the first thing he does at the start of work is run and buck, then that's the habit that will be instilled. Not great to discover this the day you can't lunge for some reason.
The other problem with lunging is that the horse isn't building his top-line muscles that he needs to carry us. He also isn't practising softness in the bridle and self-carriage.
Long-reining addresses all of these issues and is a really useful training tool.
I long-rein all of my horses as soon as they are old enough to wear a bridle (have been seen by the dentist) and are around 2.5 years, or earlier if I think they are mature enough, and could benefit from the mental stimulation of bridle work and the physical development of their top-line.
It's a wonderful warm-up before work or for a horse that you don't know well or has been out of work for a time. You can stand safely in the middle and check that the horse is engaged in the lesson, relaxed, soft and ready to ride - all good to know before you get on!
The main benefit over lunging must be the fact that the horse is being educated while long-reining rather than worked down. The problem with 'taking the edge off' is that when you do mount and go to begin your lesson, the horse is already tired and therefore not learning optimally.
Energy is our most precious commodity, don't waste it!