... a debate as heated as treeless vs. tree." (Apologies to Joyce Kilmer.)
I ran into a really interesting topic on one of the bigger bulletin boards today, comparing saddles and bras - it was such an unusual juxtaposition (to my mind, anyway) that I just had to read it. The original poster was comparing treed saddles to bras made of wood (!!!), and saying that treeless saddles are far better.
Now, as a saddle fitter, I obviously have an affinity to treed saddles, since one of my sources of income is adding or removing flocking to adjust the fit of one. But I'm not a hard line anti-treeless type; my main objection to treeless saddles comes when there's no clearance for the spine, and the saddle sits directly on the spinous process - not a good thing. There are treeless saddles out there - Barefoot and Heather Moffet are two - that have a system of panels with a channel between them, and sometimes they're a perfectly adequate answer to a horse's fitting needs. However, they do tend to lack lateral stability on some horses, and the lack of a tree makes it pretty easy for an asymmetrical horse and/or rider to moosh them out of shape. But if they work, so much the better; I'm of the "if it ain't broke, don't fix it" school of thought.
When it comes to saddles and saddle fitting, I often compare it to shoes: "Getting a saddle with far too wide a tree and thinking you can make it fit by adding pads is sort of like getting a pair of shoes 3 sizes to big and planning to make them work by wearing two pairs of thick socks," or "Trying to fix the fit of a too-narrow tree by adding pads is like trying to fix the fit of too-small shoes by adding another pair of socks." While I do think those are accurate analogies, I think fellow saddle fitter Galadriel Billington's (http://lorienstable.com/) comparison of saddles and backpacks illustrates the treeless vs. treed issue very well.
I've hiked many, many miles carrying both framed and frameless backpacks. A frameless pack is fine for a day hike if you're just carrying your lunch, water and a few incidentals, but for longer hauls with bigger loads, I like a pack with a frame. Instead of allowing the entire load to drop to the bottom of the pack and hang just on the pack straps, the frame allows the weight to be distrbuted vertically and more evenly between shoulders and hips.
To carry the analogy even further, I'll use the example of having spent many hours lugging my sons on my back. I've carried them piggy-back, in a sling, and in a Gerry pack with a frame. Again, I found it easier to carry a listing, wiggling, jouncing 30 pound toddler in a framed pack than any other way. (And while I'd like to think that I'm not quite as unstable in the saddle as my wee ones were in the Gerry pack, it did give me a tremendous amount of empathy for what my mare must go through when I'm sitting unevenly.)
Of course, there are positives and negatives to both sides of the treed vs. treeless issue, as there are to most things. I've seen ill-fitting treed saddles do serious damage to horses, and I've seen ill-fitting treeless saddles do damage, too. I've worn frameless packs that rubbed my shoulders raw, and worn framed packs that wore holes in my hips. If the basic overall fit isn't correct, there will be problems, whether there's an internal frame or tree - or not!