A pilot whose certificate I just looked up has both a valid second class medical and a BasicMed CMEC date listed. Why would a pilot with a valid medical use BasicMed?
Any number of reasons.
It could be that their medical expired and they were operating under BasicMed until they could renew it. It could be that they are planning not to renew their medical and just operate under BasicMed once it expires. It could be that they want to still be good to fly if their medical is deferred in the future. It could even be that they just want to be covered in case they misplace their medical and want to keep flying until the replacement arrives.