Red....this is a great subject to bring up.
I run a school and business and we hardly ever use trainer kites anymore.
If you run a student centered approach, you'll realize that trainers help, but not as much as people think. Student centered means doing what is in the best interest of the student in comparison to teacher centered which is what works best for the teacher or biz.
Let me explain:
1) The hardest thing for new riders to learn is sheeting in and sheeting out properly. Without a doubt this is the biggest hurdle for flying kites for beginners. Learning on a 2 line kite doesn't teach this. Most everything that you can learn with a 2 line on land can be taught properly in the water if done the right way.
2) Learning the basics of flying a kite with a 4 line kite is actually quite easy if you teach the proper way. It will take a little more time on the water to learn, but save much more time by bypassing land training. You can teach left/right turning by just starting out with the kite on it's wing tip on the water and bring the kite up to 45 degrees and back down. Do this over and over until they understand. Don't even come close to going overhead until they understand the right and left at the side of the wind window and are starting to understand sheeting in and out on the bar.. There's very little pull and it gives them an easy out to let go if needed.
3) Size down one size kite and that will work. If it's 9m winds, size to a 6m. If it's 12m winds, size down to a 9m. You get the idea. It's not hard at all training in most any wind conditions except strong winds for new riders. You must be cautious though on how they are picking it up. Go too fast through the progression and they could slam your kite more.
About the only time we use trainer kites is on snow and hard icy surfaces or for students who are not sure they want to learn kiteboarding and just want to see what it is like. If a student is really nervous, that might be another option as well as some who may not be physically up to the task.
Hope this helps.