Hardened valve seats - do they really matter?

On the head in question that I brought up in this thread that I had insisted they put hardened seats into at least on the exhaust side I did put oversized valves in. And my understanding was that with surface hardening, they'd have machined thru the surface hardening. Which was why I wanted new seats put in.
But on the other side, the valves I put in were stainless, of which from what I understand in itself goes a long way towards preventing valve and seat burning. So there's an argument right there on each side of the fence....but given the money already spent on the overhaul, the amount extra it cost for the hardened seats vs not putting them in, was cheap insurance the job wouldn't go sour