An often cited drawback to the nature of channeled spells such as Arcane Missles or a shadow priest's Mind Flay is that the mana expenditure is up front. Which means, you spend the mana and then have to stay standing in one spot, uninterrupted for x-amount of time in order to reap the full benefits of the mana you just spent on it. (I feel your pain, shadow priests of the world ~ having to rely upon a channeled spell heavily as a source of dps isn't always so hot under a lot of circumstances.)
These days, with fights getting longer and bosses with hit points that measure in the millions, mana efficiency is as big a deal as ever. Which is why it's pretty important to figure out how to maximize not only your damage output, but within the parameters of decent mana management.
An often overlooked aspect of the "nuke"-type spell (one which I'd classify as one with a longish 2+ second cast time followed by a travel time before the damage is inflicted) such as fireball or frostbolt or shadowbolt, is the travel time. People don't think about it. And while I think the overall impact to dps is probably pretty inconsequential and hence the reason why people don't factor it into dps at all, it can have impacts upon mana efficiency, and dps as a result as well. Because of the travel time ~ the hidden factor that does not make it true "direct damage" by at least one definition.
You probably know what I'm talking about. A mob is getting lowish on health, and as you wind up to nuke them down one last time... you cast the spell, and as it leaves your fingers and heads the monster's way... it's health vaporizes from all the focus fire of so many other people pounding away at it and your spell lands with a big thud. Not a biggie. It happens all the time. But what you did just do was spend the mana for a spell that actually got cast, but... didn't do anything. Bye bye 400 mana, or whatever.
Honestly, it happens a lot on trash. And ultimately, who cares if you waste a little excessive mana on trash?? You're just going to sit down for a drink right away anyway.
My point? Well, ultimately not much of one, since I have to admit I'm prone to theorizing about things just for the sake of theorizing about them. Clearly any dps-conscious caster is well aware by the time they reach, say, UBRS that a wasted cast time is wasted dps. My point is that sometimes it is wasted mana as well and not only in the better known circumstance of channeled spells. Where this matters mainly is in that many bosses have adds... and when those adds come it is invariably a mana and time sensitive matter.
I suppose I would propose that another class of offensive spell exists that unlike channeled spells or mid-long cast time nukes offers a failsafe from wasting mana in relatively small windows of opportunity, such as the adds on bosses often present. They would be... "blast" spells, I suppose. Or "burst". Arcane Blast, Fire Blast, Mind Blast, Scorch, Shadowburn, you know, those spells. The beautiful thing about them is that they are the class of spell that does not pay mana up front before the damage is done. True "direct damage". They are light-weight, quick spells that pack a wallop without any travel time and are incapable of wasting your mana (even if their DPM ranges from great to highly debatable). I like that. :)