It creates a vulnerability for external devices -- they're trying to communicate with myhost.mydomain.com but really they're talking to whatever device has the

It creates a vulnerability for external devices -- they're trying to communicate with myhost.mydomain.com but really they're talking to whatever device has the same IP address on the network they're attached to. Some DNS hosters disallow private IPs. Some public resolvers and consumer routers will filter out responses containing private IPs.