2.  In order for a saddle to move with the movements of a horse's back, 
it must have some flexibility to it.
Ideally, a saddle should combine the right amount of rigidity with the 
right amount of flexibility.  What these right amounts vary on a number 
of factors.  
a)  the weight of the rider
b)  the fitness of the horse
c)  the conformation of the horse
d)  the riding style of the rider
e)  the activity pursued
.....there are probably more.
These things vary from horse to horse and rider to rider.  The right 
rigidity of tree will vary as well.
For me, I have found, because I am quite light weight (don't need a lot 
of weight distribution capability) and the types of horses I ride, 
generally fairly high withered (I would consider a mutton withered horse 
to be disqualified as a competitive mount for me, since I don't want to 
change my saddles), that a nice, well-made, properly selected wood (teak) 
tree in a hunt/jump/cross-country english saddle works best.
This provides the right combination of rigidity and flexibility for me 
and my horses.  For those of you who don't think that wood is flexible, 
just go out and take a look at your garage door some day...wood has lots 
of bend in it.  If the saddle maker knows what he is doing and selects a 
piece of wood with the right grain pattern and builds his saddle around 
this piece of wood, understanding just how it is going to flex and bend 
under the pressure of the rider and the forces of movement from the 
horse's back....well, then you have a good saddle.  And THAT is what you 
used to pay for when you bought an expensive saddle (in addition to the 
quality of the leather).
The new fangled synthetic substitutes found in english saddles of recent 
manufacture do not impress me.  Give me good, old fashioned, well-crafted 
wood any day.  Not for me that "spring tree"
But that is just me....and my horses.  And now...if I were in the market 
for another horse, I would get one to fit the saddles I already own.  
Rather than doing it the other way around.  But that is also, just me.
kat
Orange County, Calif.