|WhatsApp (Photo credit: abulhussain)|
You can attach anything to an email. not only pictures, sounds, app-links, locations, but also any type of file. It shows nicely on your device, with pictures and well formatted. A yes formatting. You have plenty of possibilities. You could sent it as plain text or pimped out like a complete webpage. You have virtually no character limit, and filtered. In GMail you can select if a message is important or not. It recognizes those mails. And you get it on-the-go directly in real-time. Like a SMS. You can reply within a conversation thread. And if you're at home, you have all your messages directly in your Webmail-UI. With endless possibilities to sort, prioritize and labeling. Or just leave it, archive it or delete it. You can sent to a massive group of people, or very private, to one single person.
The key function here is, that on modern smartphones with push-mail functionality, like Android (and sure most of the others), you'll get those mails directly. No wait. You can use it like a chat. Of course if you want to fire up a quick short chat, you could use GTalk, which is XMPP compatible and available on any (!) official android phone or tablet.
But for email, there is another point that beats everything else. Everybody has an email-address. At the office, at home, you can check it on the laptop or desktop using a shell program without UI, or a super-lickable shiny app with colorful buttons. Or real-time on mobile everywhere. And it's available on any smartphone.
And now again tell me, why I should waste space, cpu-power and battery for a service that is already on my phone... on steroids?