There's loads of good answers in this thread but one thing that seems to have been overlooked.
Most answers seem to be related to using discs on the course rather than the practice field.
On the course I take a full bag of different discs ready for any situation - I might pull out a high speed driver for a 100' shot, if that shot is to get through a 2' gap under foliage, skip on a dirt path and pop up towards the basket for that got to make 2 (it will almost certainly not work but in matchplay it might be the only way to try
) I can do strange skip upshots with my Boss I couldn't do with my Teebird
On the practice field though if working solely on form that is where the putters/mids and fairways come out, preferably a number of the same mould These are much better for working on form as they are consistent, so if something changes in their flight it is because YOU changed something dramatically enough to affect an otherwise stable consistent disc. Doesn't matter if that change is good or bad, you know it was you that did it. With high speed stuff, the change can be tiny and have a dramatic effect and not give you good feedback. Worse still is you will work out ways of throwing them that will work but will not allow your game to develop further as it will produce bad form habits. After years of playing if I am doing form work I try to avoid faster discs, I used to use them for form work and wish now that I hadn't
It's also much easier to snap a smaller rimmed disc as you need less grip strength to hold onto it through the pivot. Stronger snap gives you more spin and more control over the flight of a disc.
Basically, learning good form is easier with slower discs and much easier if you attempt to do this in a field rather than on the course.
Most people though will be perfectly happy never practising and hitting the course up with their bosses and nukes throwing OAT'd 250' drives and producing bad form which will never let them throw a putter straight.
And that's fine too.