It's totally possible to run out of mana, unlike what some people are saying. In Titan during the stomps, when you're constantly having to use Succor and use your Aetherflow stacks on Lustrate on the tank as well as people who get hit by the mechanics, you have no MP coming in. If your bard dies in those situations, you will quickly find yourself out of MP.

That being said, in easier content the only real way to run out of MP is if everybody is getting hit by everything, and aggro is everywhere, and you're stuck spamming heals constantly.