If you don't re-register, the same private key will be used to claim both the Evolution (EVO) and the EOS tokens. Right now a conservative person might worry that Evolution is trying to steal the private keys that will claim peoples' EOS stake. They could do this by creating software that has you enter your private key to claim your EVO tokens, and then using that private key themselves to take control of your real EOS account.
If they publicly encourage people to change their keys after the Evolution airdrop date, that gives us more reason to trust that they're not just trying to steal EOS keys.
If they were actually scammers (note, I have no idea if they're legit or not, but with security always assume the worst), we might expect them to go out of their way not to educate people about the risks.
RE: Evolution Airdrop - May 10th