I find myself using the Character Map (or Keyboard Viewer) all the time, because the keystrokes required aren't intuitive, and aren't labelled on my physical keyboard.
So for some users, press-and-hold on "e" to get é is far more efficient than having to open up a separate application entirely.
For that matter, I really like OS X's approach where you can use either input method.
Likewise, you can always switch to the specific language's keyboard on iOS, and press and hold is similarly obviated.
I don't know, I've always found the Word keystrokes intuitive (in fact that I was able to discover most of the ones I needed for French and Spanish by learning one and generalizing from there), and my main frustration in that regard is that they aren't standardized for text input in Windows.
As for labelled -- if I have to look at my keyboard to type, that's too much of an efficiency killer to start with.
> Likewise, you can always switch to the specific language's keyboard on iOS, and press and hold is similarly obviated.
Except that its generally not; even with the Spanish keyboard, you still need press-and-hold to get to the vowel accents.
Language-specific autocorrect is common in desktop software that has autocorrect at all (e.g., wordprocessors) and, similarly, good access to international characters through methods faster than press-and-hold is pretty common in lots of desktop software.
Press-and-hold is a slow, clumsy approach that's only tolerable with a virtual keyboard because virtual keyboards are already so slow and clumsy that its not a big marginal cost.