Yes, it would. But people don't always do that (often they don't even
realise what the host key is *for* until it's too late, and then, oops!).
More than once I've seen disaster-recovery hosts hotswapped into the place
of the machine they replace, IP address and all... but oops! they have a
different host key! Too late to fix, the original machine is dead now.
(Yes, this is a configuration error. But it's a pretty common one. More
common than not, I'd almost say.)