Casting Cure I 30 times takes 1 minutes (no spell speed) which costs you 3990 mana. Assuming a mana pool of 4000 that would be 99.75% of your mana but during that same time you will have regenerated 4800mp ((4000 x 6%) * (60secs / 3))

Assuming a mana pool of 5000 that would be 79.8% of your mana but during that same time you will have regenerated 6000mp ((5000 x 6%) * (60secs / 3))

When using cure 1 you will be getting back more mana than you burn through. Using your other heals is what pushes you down. The larger your mana pool the more tolerance you have built in to use those other heals.

Shroud is for when you need a large number of those other heals and need to stabilize after or get caught back up a bit because you didn't weave in enough Cure I to keep your mana stable. And as your gear makes your Cure I stronger it will become a more used part of your healing toolkit because its able to handle more and more situations making you that much more mana efficient.