Is it important for "mages" to reach 45% CDR? Yeah it kind of is since they do zero damage when their abilities are down. AD champions can still auto attack and deal damage, champions that build AP don't have that any longer after the mastery changes, no more Arcane Blade, no more auto attack damage bonuses leaving any ap building champion's auto attacks, minus kassadin, irrelevant. Abilities on CD? Enjoy getting out damaged by minions.
There's never a situation where you're going to build any set of these CDR items that equates to anything above 30% CDR, it's actually so difficult to reach 45% CDR at this point in time for AP champions that it's becoming meta for them to build
to deal with mana issues, CDR difficulties, and the increased expenses of their items, which leads me into my next point.
If you look at what happened to ap items, basically every item now costs more, and/or got stat nerfs on the items, with a few very rare situations. Lichbane is now 200g more, Athene's 100g more, Liandry's 200g more, Luden's 200g more, Morellonomicon 250g more grievous wounds nerfed, Deathcap 300g more, RoA 300g more, Void Staff 150g more, Zhonya's 500g more, 5 less armor, Archangel's 100g more, Seeker's Armguard 10 less armor, mana pots gone.
What this means is the typical AP champions now requires about 1500g more to build their items, meanwhile magic resistance items now give more magic resistance, so for their increased gold expenditure, they're actually weaker.
Masteries, the new masteries are great, for AD champions, do they have some interesting stuff for AP champions? Sure, but there's two paths to take 18/12/0, or 12/18/0. You're obligates to take either Deathfire Touch, which only works on very few champions well, or Thunderlord's Decree, which pretty much commits you to the 45% CDR path. AD champions can take Warlord's Bloodlust, Fervor of Battle, Deathfire Touch in some situations, Thunderlord's Decree, and even Stormraider's Surge.
Other champions need to sacrifice a great deal to itemize into MR? Last I checked AP champions are the only champions that have to deal with an item being built that increases resistances across the entire enemy team, which nullifies our obligatory magic penetration runes, and then provides additional resistances on top of that. So sorry that you might have to invest in a
that doesn't build into anything you could possibly use.....oh wait
. What do we AP based mid lane mages get from armor that's of any use to us at all?
, which is also 500 gold more expensive now for fewer stats.