The reason this problem exists is because computers and font libraries don't have all the characters we need. The point or comma should be at the same place as the quotes, under them, not before or after.
similarly, the comma can be inherited by the formulation and implied (there is no sacrifice in '«That's too bad», John said, «but you can recover»' for '«That's too bad, but you can recover»' - the break is in the formulation and the explicit comma is redundant).
What?
It does save space, but I'd say that's neither here nor there. It isn't like space for an extra period at the end of a sentence was ever at a premium.
At best, for a whole 200+ page book, you'd save a page or so.