Why STL classes do not overload swap() for rvalues?

STL classes define swap() method as void swap(A&), taking an l-value reference. See for example std::vector::swap, or this question "Is `std::move` necessary here?".

Such definition means we cannot swap with r-values, since r-value won't bind to However, I see no harm in swapping with r-values. Construct it, steal from it, place some guts in it, destroy it. Done. We can add another overload void swap(A&&) to make it happen.

I see only one reason why we do not have this overload out of the box. Because instead of writing

v.swap(rvalue);

It is better to write

v = rvalue;

And instead of swapping we will trigger move-assignment, which is even more efficient. Am I right that this reason is valid? Is this the only reason?