Flat vs Scaling CDR Glyphs?

I dont like scaling cdr glyphs, I only put 5% and MR in the others With masteries I start the game with 10% wich is good enough

Flat CDR is more reliable when building (if you do plan on building CDR) and has a better early game impact. If you go scaling though, you get more cooldown later on and can better utilize your spells. It's a question mostly of Preference: Are you willing to sacrifice early and mid-game power for late?

I use scaling CDR Quints on siege mages. Rather drop the 15 AP over the life saving MR. Usually build this into etc. It does not lack AP.

Flat CDR, I don't really see the point. Usually, the early levels on a mage, your abilities are more gated by mana costs than cooldowns. I guess an energy champ would be interested?

I was about to make a much longer response but I decided to delete all my work...

In a nutshell flat CDR is best when your just trying to fill out your last bit needed to reach 40% cap or to experiment on champs that are resource-less . you may find a niche build for a specific champ that makes you that much of annoyance , Your turrets hurt more if enemies deleting them from the game is a waste of time right ? keeps them buggers busy !

Overall, I think they have great merit but, they are mechanic and play style dependent. Building them makes you loose out on potential defense, damage per spell cast or on auto, and mana regen. Build accordingly

Scaling CDR are already explained well.