Sorry guys - Im going to have to pull rank here.
From my limited time running a school I can say with absolute certainty that flying a trainer is DEFINITELY a great way to go for kiting - esp when starting but even beyond.
YES I agree flying ur LEI seems quite different but its not - just slower. Its all the same kite control. Flying a zippy little trained - esp if you work to a program - rather than just whizzing it around aimlessly - will develop all the mind & muscle memory needed for skillful kite flying.
I have tested and observed lessons objectively for all of this summer. What I see is kiters being pushed along by instructors who are very conscious of the time/cost factor. If you are pushed thru to advanced stuff - body dragging, board on etc while you are still struggling with kite control there are 2 consequences
1) you have students stuffing up the learning part of the lesson because they are not flying the kite properly. NOT because they cant get the body position (or whatever ur teaching) right.
2) you have students so intent on trying to control "this monster that might kill me any minute" unable to 'hear' the instructor. Nothing shuts down your attention, concentration and ability to hear & learn more than nervous overload.
We had one student (in particular) who was going no-where at the body drag stage. 2 x lessons just trying to get her suspended in the water preparing to body drag. Sent her off with the loan of trainer and a set of exercises to work on. She did 20+ hrs in a week before coming back with every move and action fully dialled on the trainer. She then went thru all the prep lesson stages and was up & riding in just one more lesson.
Your experience may be how it was for you but its not fair to put other ppl off what is proven to be beneficial.
At Woodies we get the No1 world champs every summer training at our Beach 3 flat water. What do those guys do when theres no wind ?
They fly their trainers - for hours on end.

As my instructor told me when I was learning - "Kiting is all about flying your kite"
Especially those who are concerned about the cost of lessons should ensure they get full value by learning to fly a kite first. Borrow, beg or steal but its THE BEST way-to-go. (No dont steal!!!)