Chaining inserts ... This would be cool

There are obviously workarounds for this, but I'm wondering why the
following query shouldn't work. It seems like it should. With MVCC already
present on the back-end, I can't see any reason other than additional
parsing routines that this couldn't work:

INSERT INTO old_login_id_to_new_account_id(new_account_id, old_login_id)
INSERT INTO accounts(id, username, password_hash, email) SELECT DEFAULT,
username, password_hash, email FROM logins_old RETURNING id, logins_old.id;

Anyway, I'm sure there are more important features for Postgres (like
upserts, unique indexes on GIN, Gist and hash, fixed arrays, compact
storage of enum arrays as bitfields, etc.) I just thought it was an
interesting idea.