Well, out of combat everyone has food, out of trouble there are bandages. But Invocation heal is handy to top yourself off. It's especially good when you solo old content and there's a boss with special mechanics that make you wait for something while taking damage. Or for example Yogg-Saron - you have to kill adds first, to make the boss appear and if you want to do an achievement, you have to kill 9 of them almost simultaneously. After that you're kind of beat up, but during the phase transition you're free to chain-evocate to full health.
Invocation heal, despite being rather weak, is the only useful utility of our t90 talents.