For one thing, in SQLite, it doesn't. Which is a weak argument for not using it on supported systems. The other weak argument is that a RIGHT JOIN is just the b, a version of a LEFT JOIN a, b.
When you add them up it's an extra concept, SQL execution flow is already somewhat unintuitive, and a policy of using one of the two ways of saying "everything from a and matches from b" makes for a more consistent codebase.
I would hope a blue-sky relational query language wouldn't support two syntaxes for an operation which is non-commutative, when order is important it can and should be indicted by order.
