Um, if it isn't obvious, there is also the audience to think about.
Also, it is not simply a question of whether one artist will get annoyed if you do this on one gig - it is another way of beign a nuisance (if you can see the assist light on the artist - which you would for it to be effective - it is a nuisance to some extent, yes?), and this will lead to 2 and 1 song rules and so on. Since there almost never is a guy who makes sure the press behaves and is educated about assist lights etc, they'll just thigten the noose for everybody. And I'm sure it is enough to annoy some artists only once and after that it's one song rule for everybody.
This is of course a generalisation. For instance, some punk bands (playing at a small club) / their audience couldn't care less about such things (I'm sure).
Also, since the assist light can be used only in one-shot mode (if I'm not mistaken), isn't it anyways pretty easy to get a focus lock on a static subject when there is enough light to photograph him/her anyway?
As a side note, I've been to a few concerts where it was oblicatory to take the flash off camera before going in (this was at a festival so a number of people actually had flash on their camera for fill light for previously shooting general audience stuff or other bands).