I think any player, young and old, weak or strong, should start off with just a putter. Once they start to max out the distance possibility of that putter (around 300') add a mid till they hit 400' and so on.
They'll definitely achieve more control over a shorter amount of time this way and that is the important part.
I agree with you about the putter start, but most people cannot throw a putter 300 ft. I throw an r-pro boss over 400 ft, but max out with my challenger at okay, well almost 300. But I think maybe at 150 ft with a putter they could benefit from an understable mid or even a driver.
I donno. I find that throwing a putter 250+ doesn't require a whole lot of effort. You just need to find the line and regardless of your power level you can lay a putter up there pretty far. Heck, if you torque a putter really hard it's just going to flip over on you anyway. I think that people move up mostly because they see an easy distance increase and don't consider that maybe they can build more skill by sticking with the putters and mids.
The important part of driving a putter being that you get it up there on a straight line that stops short instead of a giant hyzer which may not be as predictable and can skip past the target.
I definately agree about your suggestion being the best way to learn disc golf. The mids and putters make up your core game and should get most of your attention (until you're a ROC solid AVIAtoR, or the market equivalent).
I don't think a person should shy away from drivers altogether before then though. Drivers are fun to throw. Spend the bulk of your practice on putters and mids, but then loose a few drivers too to spice it up. But those drivers are addicting and I think most people I play with and often me as well get caught up in the driver craze.
You offer some good advice...not just for the beginning golfer, but for the rest of us ams as well.