Not guaranteed that it will transfer overnight. Even if it usually works, on that random occasion that it doesn't, you have no back-up plan.
And not every LH account has a matching SPG account with it.
Let me explain.
I'm very upset that this option is going away.
Most of the time there is no spg in the AH name, which makes this a great option to top off an account.
But, when there is matching spg, almost always I would transfer it instead of buying if I can put the ticket on hold or there are a bunch of seats open.
Makes sense?