A cookie cutter spec is not the end all, be all spec.
A cookie cutter spec is a base line spec that will serve you well in any situation. It is an unmodified car, the basic WoW UI with no addons. It will get the job done.
A cookie cutter spec, however, is not always the best spec.
A raiding fireball mage, for instance, does not take blastwave or dragon’s breath. But… what if that fireball mage raids 10-man and is the only non-healer ranged class in the raid? Suddenly those two talents become extremely good for the Deathbringer Saurfang fight, don’t they?
The cookie-cutter arcane spec, for instance, does not include Slow. But again, consider the Deathbringer example above. What if this mage doesn’t raid, but instead chain runs heroics? Slow is amazingly good in any kiting situation, whether that is due to a fight’s mechanic or tanks incompetence.
Sure, you could use frostbolt instead. You could very easily simply use a spell that takes around a second and a half to cast with a 20% weaker snare. Slow is better, of course, and will make the fight far easier, but it is still possible to be victorious without it.
Some talents are indisputable, you always take them. Things like Empowered Fire, Summon Water Elemental, and Netherwind Presence are always taken, no matter the situation.
But then there are other little talents, things like Incanter’s Absorption, Magic Attunement, or Frost Warding. These talents are not useful in every situation, and so are disputable.
Ulduar contained a lot of raid-wide damage. For a great many of those fights, that raid-wide damage took the form of frost damage and fire damage. Combine this with the fact one of our main healers was a discipline priest, Incanter’s Absorption and Frost Warding were incredibly good talents for me to take.
For instance, I’d see Ignis casting Flame Jets (or whatever the interrupt is called, it’s been a while), cast Fire Ward, and I’d get anything from about 6k mana instantly restored to a huge boost in spellpower. Either way, it was an immensely powerful talent.
Trial of the Crusader saw similar fights. Beasts and Jaraxxus provided ample opportunity to use Fire Ward, Twins essentially gave me a consistent extra 1k+ spellpower for the entire duration of the fight, and Anub would often lavish me with mana and spellpower via that frost damage debuff thing he does.
However, Icecrown has been… somewhat less friendly to the IA department. There is still an immense amount of raid damage, but so far very little of it is frost or fire based, and when it is, it is rarely predictable. Marrowgar is the only fight so far where I can get consistent use of IA/Frost Warding, and frankly carrying around a spec for a single on farm fight is simply silly.
Moving forward into the Plagueworks, there is a HUGE amount of raid wide spell damage, however none of it is frost or fire based. Most of it is nature and shadow based. Thus, the talent Magic Absorption suddenly comes to the forefront as an amazing survivability talent and a solid source of mana.
In addition, there is also a HUGE amount of emphasis put on movement in the Plagueworks fights. Well, mostly Putricide. Thus, bumping my main nuke’s range up to 36 yards suddenly becomes a massive increase to DPS. More range, of course, means DPS can start sooner, meaning more DPS.
As such, I have adjusted my spec to accommodate these changing raiding demands.Incanter’s Absorption gets dropped in favour of Magic Absorption and another point in Magic Attunement, and the two points I had in Frost Warding are shifted into Ice Shards. (Frost Warding has been, so far, completely useless, but Ice Shards is a considerable boost to trash DPS.)
There is nothing inherently wrong with using cookie cutter specs. Such a spec will satisfy the needs of any encounter, but this does not mean that it is the best spec to use for any encounter.
Try to remember that specs are meant to meet YOUR needs, for the content YOU are doing. Your needs may be very different from mine or the imaginary mage rawr uses.