Signature is the best option to do marketing of your brand. You can convey your brand message to everyone whom you will send your message. Its a best and easy way to send your brand slogan to others.
whatsapp is a social application. You can use whatsapp for your brand marketing but right now message signature option is not available. Its my personal opinion that whatsapp should allow this activity on this platform. More then 60 billion messages are sent through whatsapp.
Hope so might be they are working on it and hope so this feature will be available very soon.