Those who start on the sax often have a problem with "hitting the holes" dead on. Playing the sax, with nary a finger hole to be seen on the horn, does not really force the player to adopt the exact finger placement that clarinet players have to learn from the get-go.
I've even known a few sax players to routinely place their fingers off of the pearls (as seen by the lacquer wear on the key pad cups). As long as all the linkages are enabled, you can get away with that on a sax. Not so on the clarinet.
Finger hole leakage (after poor seals of the key pad overall) is the most common cause for squeals on the clarinet. The slightest inperfection of the location of your finger is all that it takes to open a "register vent" in just the wrong location, and a squeal follows close behind. Some have used the plateau horn to get around this, and it works but with the price of veiling the tone, at least as heard by the player. (Others who are listening out front don't seem to notice.)
I have also known an older fellow who had a broken ring finger with a badly crooked first joint (displaced towards the little finger) on his right hand. As a result, he had a lot of trouble playing the clarinet. When he got a plateau clarinet, all of those problems disappeared. Unfortunately, he died soon afterwards.
A better approach for the sax player moving over to the clarinet would be to purchase a Vito student clarinet. These have (or at least had, the last time I bothered to look) smaller fingers holes, the better for a less-experienced player. You can get these horns for next to nothing, and they will "train" your hands to center the fingers on the holes without having to suffer through months of trouble. Do your learnin' on this inexpensive Vito, and after you have mastered the finger placement (and decided that clarinet playing is something you want to do), you can step up to a pro horn without worrying about if you can manage it.
