If a Caster DPS shouldn't have to worry about mana, why have the resource in the first place? I guess that's why Destro warlocks have over 600% mana regen.
First off there are two 'models' of design; global capped specs and resource capped specs. Melee classes, Hunters, and Destruction Warlocks are resource capped - they don't need to fill every global cooldown to maximise DPS. Global capped specs do.
Resource capped classes tend not to worry about running out of resources, instead they worry about capping them; be it Rage, Energy, Holy, or Runic Power (even Focus for Hunters). They can DPS forever, casters executing their normal DPS rotations can, and should too. Running out of mana because a fight dragged out too long should not be a concern for casters when it is not one for Melee/Hunters/Destruction Warlocks.
On Destruction Warlocks' mana specifically, it works like Energy; in simple terms you spend it to generate Embers, and use Embers through Chaos Bolt and Shadowburn to regenerate mana. You don't want to cap your mana bar, because it's a waste of resources for when you wish to move and need to use Fel Flame which is very expensive. The spec is designed to be resource capped, like melee DPS, rather than GCD capped as other casters are and as such needs to manage its resources more than any other caster to be able to DPS effectively. It is a very, very poor example to use of a spec for which mana doesn't matter and shows a complete lack of understanding of the subject. Perhaps they should have gone ahead and changed Destruction's Mana bar to a Fel Energy bar.