The 'A Different Voice For Each Character' Myth

I've been spoonfed to the point of sickness the advice to make each character sound distinctive in terms of what they say, and how they say it.

But wait a minute. Of all the novels and screenplays I've ever read, there's been very little to distinguish one character's dialogue from the next, unless they have a stutter, a really foul mouth or an absinthe-strong dialect.

And is it really that important that each character speaks in a noticeably different way? Is a book really spoiled by having characters speak in a similar way?

The more I think about it, the more this sounds like another of those myths that theorists trot out to fill space in their 'how to' books, and one that pro writers just plain ignore.



(There's another myth of writing here, in case you mythed it.)