I did not imply that skinny people did not hurt the horse's back. I simply said that if you are going to ride in a treeless you should be skinny. I would hope that every rider has help riding correctly. And I do understand the science of weight distribution ;)
I am 260 lbs and I successfully ride in a Bob Marshall endurance saddle, which is treeless. I have been riding in it for nearly two years now and my horse's chiropractor is THRILLED with the condition of her back.
I use a specialized Skito pad
designed for treeless saddles and a ThinLine pad
underneath. My weight is distributed well and my horse is more comfortable than she was in her custom fitted saddles. We ride anywhere from short 30 minute arena/pasture rides to 4-5 hour trail rides. Our trail rides, whether it be 1 hour or 5 hours, consist of flat and hill work, walk, trot, and canter work. We take breaks when needed.
Both of my horses show less fatigue and "grumpiness" at the end of our rides with the treeless than they did with a properly padded pro-fitted treed saddle (western or English).
The reason some horses may be "hurt" by a treeless saddle include improper fit (yes you DO need to "fit" a treeless saddle), improper padding underneath, improper riding (I am a very balanced, long time rider), a cheap poorly made Asian saddle, or a combination.
You can't just buy some cheap (or not so cheap) treeless saddle off eBay, slap it on your horse, and expect it to be perfect. As with any saddle purchase, you need to do your homework. Different saddle work for different horses and riders. Padding becomes VERY important with treeless saddles. There are many brands of pads to choose from. The first one to start with should be the pad recommended by the saddle manufacturer, or a Skito pad (always a favorite among treeless and treed riders alike). Girths and breastcollars can be tricky as well. To the OP:
The best way to begin a treeless journey is to start with a retailer that offers demo saddles. Demo the brand and model that sounds like it will work best. Be sure to get pads, shims, girths, and all the extras you'll need. Send in pictures and details of your horse to the appropriate "goodies" will be sent with your saddle. Try to demo during a period where you can ride daily, or at least 3-4 times, so you can get a real feel for the saddle. Both horse and rider need to "get used to" the new feel. Some horses take right to it, others need some adjustment time. Keep a notebook with your thoughts of each ride and your horse's reactions/actions during and after each ride. Talk to your dealer if you're having any issues. Sometimes just a different placement, girth, or shim can "fix" an issue.
For high withered horses, I would look at Sensation, Freeform, Ghost, Trekker, Heather Moffett saddles, and a "peaked" Bob Marshall saddle. Each brand has many models. Talk to the dealer of your choice to decide which one will be best for you and your horse.
There is a great group on Yahoo with treeless afficianados and dealers. They can be very helpful in deciding which treeless brand and model to start with. The members there have all kinds of horses and most have tried two or more brands/models of treeless saddles. You're bound to get some good feedback. There might be members near you willing to let you try out their saddles as well. treelesssaddles : treeless saddle discussion
Good luck saddle shopping!