Forgive my cynicism, and thanks for replying.
I'm talking about later, once the EVO and EOS chains are operational, not now as people are registering keypairs for the airdrop. Once the Evolutionos chain is running, how will people claim their EVO tokens? Their private key will have to interact with your software in some way. If they haven't updated their EOS keys after May 10th, they'll be in the insecure position of having a single key control balances on both EOS and EVO. It would be fairly easy for you to steal peoples' valuable EOS private keys when they claim their EVO balances.
By telling people to change their EOS keys before claiming their EVO mainnet stakes, you have the opportunity to publicly increase your credibility -- as this isn't something a thief would do.
RE: Evolution Airdrop - May 10th