I got the leather ones on my customs several years ago
Because that's what I had ridden in ages ago when I had my first pair. After riding for eons in Ariat paddock boots, I now wish I'd gotten a similar sole to the paddock boots I usually ride in, plus the leather soles seem thin, and get scuffed on gravel, can't hose the mud off, etc. I save mine for shows, but it does feel very different. So I either ride in my tall boots all the time, or I deal with the fact that there's a 1/2 inch difference when I ride.
My next pair won't have leather soles I don't think.
There is a lady at my barn here with leather soles on her boots... I find it hard to decide if its her walking or a horse walking down the aisle when I hear her coming... her boots are sooooooooooo LOUD!
My old foxhunting friend said leather soles keep your feet warmer. I feel that as a dressage rider (read- diva or Dressage Queen) that I'm not out in the cold quite long enough to reap the benefits... not to mention that smart wool socks accomplish amazing things.
I got leather soles when I got my Dehners, I wished I had gotten the rubbers ones instead now. Dehner says they are pretty easy to resole, so I am just waiting for the leather to wear out and then I will replace them. I can tell you they are not as cushy or shock absorbing as the rubber. I had to put in a pair of gel footbeds, which made them very comfy so they are fine now. Next pair will have rubber soles.
I don't care for leather soles either (because of the upkeep and care they require and you feel every little thing through them while walking). My Mountain-Horse tall boots have the SCS (Stirrup Control System) which I absolutely LOVE!
As to my western show-boots: I dyed the crepe soles dark, so that I don't have to wear boots with leather soles Sure you will love your new Cavallos!