Isn't sending a message or a photo via WhatsApp the same as sending an iMessage via an iPhone though?
I hardly ever use my phone text allowance, as any messages I send to people who have the iMessage function set up (which seems to be most of them) goes through that way, not as a text.
Indeed.
But Whatsapp is platform independent so, for example, you can send a message to 20 people simultaneously regardless of what phone type they have - iPhone, Android, Windows Phone and even Blackberry. This really comes into its own for pictures where, if a recipient doesn't have an iMessage, you might incur an MMS charge which are often 50p a go or similar.