They are also common with those players who have lost portions of (or have malformed) fingers. It's not quite as common these days, but prior to the advent of modern medical procedures, a mashed up fingertip was not all that uncommon.
(Such modifications to musical instruments were quite common in the 1920's and 1930's, with far more deformed hands coming back from The Great War, when plastic surgery was in its infancy. There were also a rash of "Concertos for One Hand" during the period, with modern composers writing them to suit their piano playing friends who had lost a hand (or worst) to the huge shell fragments of the day.)
Nowadays, the Leblanc company (since absorbed by Steinway Selmer) make/made them for a different purpose. Marching band folks sometimes put their musicians in gauntlets, and clarinets were the great exception to the "smartly dressed band" rule, as they either had to go glove-less or (even worse) cut the fingertips off of the gloves. No such problem with a plateau horn (and closed hole flutes).
I had a "friend" (technically, he was but in actual truth I couldn't stand being around the guy) who had a broken finger in the days of his youth (1940's) and who (as a result) had the end of the middle finger of his right hand set off at an angle from the rest of the finger. He couldn't play clarinet worth a damn with a regular horn, but once he got a Noblet plateau horn he picked it up really quick.
Unfortunately (for him, certainly) he shortly thereafter died...and the widow had trouble selling the "oddball" horn.
Another friend (a saxophone player who took up clarinet later in life) bought one for the hole alignment issues. He was used to flopping a finger down on or near the touch pieces, and just had too much trouble with finger alignment on the holes. Despite my warnings that they had a "veiled" tone quality, he bought one and learned to play on it, but gave it up for the ever present R-13 as soon as he became proficient.
For the record, I always started students on Vito horns, simply because of the tone hole coverage issue. Children have real troubles with standard clarinets (and open holed flutes) due to reach issues, and a smaller hole Vito horn minimizes or eliminates these problems. Once they are up to speed on the Vito, then a move to an intermediate (Noblet or Signet back in the day) was in order, with "serious" students meriting the purchase of an LL, R-13 or Series 9.