I don't think I'd feel very comfortable fitting any electric shower without an isolator, regardless of whether it was a manufacturers requirement or not.I had a shower go faulty last year, I'd only fitted it a few days earlier and the elderly lady had just turned it off on the shower when it began to make a strange noise, fortunately, she had exited the shower when it decided to eject boiling water, followed by a cloud of steam. She turned off the pullcord and it stopped, I rang Triton and they sent one of their engineers out, apparently, it was a known fault on a number of these showers.
Another thing I seem to remember from a while back was when someone was electrocuted in a shower that had no local isolation, the coroner noted that, although it was not certain that the death could have been prevented, it may have been avoided had local isolation been provided. Now I know that most of us would probably agree that, if a shower, not protected by an RCD, and there are still a number of these in use, were to develop such a fault, it is highly unlikely that anyone using it would survive, it's not the sort of thing that I would like to have to live with, would you?