That's realy a strange thing. Where I live, Switzerland, the first thing you do after you decided to ride a bike (after you bought the bike and a helmet) is to get a kidney belt usualy, it's just normal. Speaking to peopele from UK, some of em don't even know what it is for, most are saying: useless. Seems to be some kind of religious thing...
I'm certainly not a doctor (only personal riding expiriences) but the kidney belt is saving your kindneys from vibrations on long term trips, saving yourself from unpleasant pain. It's easy, if you have have problems with it -> try it.
And there is another very important option: back protectors with integrated kidney belt (or the other way round). I know a few riders who would be dead or paralyzed without it :-( .They're uncomfortable the first time you wear it and you have to get used of it. Me personaly I feel naked on the bike w/o a kidney belt and a back protector nowadays.
Save rides, Kudi