Makiwara, or machiwara as it is called in Hogen, is probably the most misunderstood and incorrectly used training equipment in the dojo today. So, perhaps it is a good thing that most modern Karate-ka don't train hitting one. The potential damage of misusing a Machiwara could be very long lasting if not permanent. On the other hand it is the primary tool for learning and polishing proper technique. Any one sincerely interested in learning Karate will need to spend countless hours in front of it; and as a result will learn to respect what it has to offer.
Pre World War II, all Karate-ka all trained with it (see below Motobu Choki and Funakoshi Gichin). So why is it that few if any Shotokan dojo hit Makiwara today?
If I had to guess I would suggest that few sports karate people see any benefit to their training from pounding their knuckles against a wooden post. And I guess I can't blame them. Unless you understand that Tenshin, Chinkuchi and Findi are the building blocks of Machiwara training, its understandable that the thought of just standing in front of the Makiwara and pounding it for an hour or two doesn't have much appeal to it. Unfortunately, that is all that the few people I've seen hitting Makiwara are doing, just building calluses on the hands.
Most students of Karate don't realize that the sounds of the Makiwara is important, or for that matter how to bow in front of a Makiwara or when to hit. The psychological or visualization exercises that many Olympic athletes use in their training regimens have been used for generations in front of the Makiwara in Okinawan Dojo by Tijigaya.
If all you are going to do is hit it until your knuckles get thrashed, there is little value to Makiwara. If on the other hand you are taught how and for what, the Makiwara is the next best thing to having your sensei in the dojo correcting you. Just like your sensei, when you make a mistake it will teach you in short order that you've done something wrong. Makiwara is not at all about the knuckles, it about the feet, the wrists and the shoulders.