Should all Karate-ka train in Hojo Undo once in their life?

You need the right mindset and knowledge if you're going to practice Hojo Undo. Like everything else, there is a definite wrong way and definite right way. If done the right way, I don't think it matters if you use modern methods or the old ways to practice it, so long as you actually know what you're doing, especially in terms of internal activities.

Okinawan Hojo Undo has a sort of unwritten set of rules when it comes to Ki and the Tanden. It's passed on differently than in the Chinese arts, and even Japanese arts like Daito Ryu Aikijujutsu, but it's there in it's own form. People shouldn't do supplemental exercises like how it's taught in demonstrations until you have a firm understanding of those "rules".

Should all Karateka train in Hojo Undo once in their life? Aside from knowledge, I say yes. Hojo Undo is designed to strengthen the body and give fuller meaning to technique. It fills out where normal capability does not.

Hojo Undo, as meaning supplemental exercise, can go anywhere from pushup and situps, all the way to makiwara and nigiri game. Even Sanchin Kata is supplementary. So it ranges from experience, and given that experience, it's my personal believe that supplemental training should be performed.

Hojo Undo is good training. However, it does not make you stronger than many contemporary exercises do. In fact, because Hojo Undo is so specialized to affecting your performance of Karate techniques you will not be any stronger in any meaningful way.

I enjoy makiwara, chiishi, nigiri game, etc to improve my performance of kata and techniques, however I do not rely on them as my sole method of strength training. I also incorporate free weights, cardio, and other more contemporary exercises to boost my performance and skill.

Schools that do Hojo Undo are not necessarily as good as those that don't, simply that they seek to embrace a part of Karate's history.

There are exercises that have yet to be replicated or made better by science or technology, however those are exercises which are so niche not even a high percentage of traditional schools do them frequently enough to benefit from them.