Rotating the keys would make some sense, but just swapping the cert for a new one issued against the same keys doesn't. It's the easiest way to fulfill those requirements, because you don't need to synchronize the metadata updates, the signatures are always valid with both the old and new cert.
> synchronize the metadata updates
Sadly I know many implementations that do not handle key changes in the metadata in a smooth way. The two SPs I have from Adobe both require manual updating of one key per idp, making a switch pain to synchronize.
IMO, I think rotation is wildly useless too. It might make sense in a world where my signing certificate was decoupled from the metadata someone else has to very likely load by hand.