One possible explanation is that it lets the manifold "creep" around between hot and cold cycles. I really don't care as long as it keeps the big sucker off the welding table.
With regards to cast iron exhaust manifolds, an engine development engineer once told me that one of three things is always
going to happen:
1) The manifold is going to move (expand and contract).
2) The manifold is going to break.
3) The manifold is going to break something else.
I think that your solution is definitely compatible with #1.
GMC 270s are known to crack manifolds from time to time. When I rebuilt mine, I made sure that both the contact surfaces of the retainers and washers, along with their corresponding surfaces on the manifold were smooth. Not sure how much this will help - but hey.... it couldn't hurt.