It's a heck of a bit. I used to have one for my 17 hand perchie cross....and it was too much bit for him. he'd just stick his nose on his chest and plow through it. I had one of the short-shanked original Mikmars. Think I still have it somewhere - haven't used it in forever. SHould probably get around to selling it. Does this help? I would think that if you had a horse that got VERY strong and pulled with his nose straight out (think racehorse), or stick his head up in the air and just run, then it might work. Horses that drop their heads and pull will probably do what my draft cross did and just plow on through.
"Opportunity is missed by most people because it is dressed in overalls and looks like work." - Thomas Edison
So, the Zen Buddhist says to the hotdog vendor, "Make me one with everything."
While the mouthpiece looks like an instrument of torture, it's actually pretty mild. I've used a Mikmar pelham and a Mikmar short shank (no nose rope) on a couple of horses and found that they accepted the mouthpiece well. Some horses really like the roller. I tried the Mikmar training bit a long time ago on one horse but he didn't work well with the pressure on his nose.
I also have a Watson mouth loose ring snaffle which is nice because of the way it drapes in the mouth (no nutcracker effect) and leaves more room for the horse's tongue.
The size of the port on the bit will influence the pressure it places on the horse's palate. Some horses react better/worse to it so it's kind of a trial and error thing.
It's a very lightweight bit and some horses prefer that over a heavier bit.
When you add the nose rope (training bit), you are creating the opportunity to influence the horse through pressure on its nose. For some horses, adding this element allows you to control a horse without as much pressure on its mouth; other horses may take offense at the pressure on the nose. The length of the shank plays a big role here in how severe the pressure can be over the nose.
I think they can be very useful but that they are a bit overpriced (as so many bits are!). I've always bought mine on eBay.
I know several people who hunt in the short shanked bit because they find that their horse will accept the contact but it gives them enough control. However, how your horse goes in one of the bits may vary -- how well a bit works for you depends on the conformation of your horse's mouth, what type of pressure they respond best to, and how you ride. It's hard to make a general statement.
I used the Mikmar combination bit with my double reins attached to the shank and the rope. For my horse, it worked very well because he respected it without being afraid to take the contact. That said, it is a heck of a bit and too much for most horses. At home, I still jumped in a loose ring snaffle or a pelham, but with the adrenaline and excitement associated with cross country, sometimes you just need something to get the job done.