Originally Posted by RiotTiza
11% more damage is 0.11 CDR scaling. Compare that to Vlad's AP ratios - 0.6 on Q damage, 0.25 on Q heal, 0.45 on E damage, 0.7 on R damage.
CDR isn't useless, but AP scales better in terms of raw damage.
Raw damage, yes, but as Vlad you don't too particularly care about that. Your spells are somewhat low cooldown to start, and you want to keep using Q and E to recover more health and deal more damage.
In general, for just damage, CDR returns whenever you have an engagement window of a length that would let you cast one or more additional spells than without that CDR. On a 1 second spell comparing max CDR to no CDR, you're looking at windows from 0.6 to 1 second, then from 1.2 seconds on you're always at least one spellcast up on the no-CDR spell version. (The poster spell for this, Lay Waste, doesn't actually want CDR in this case because Karthus' ult CD is stupidly long)
Also, CDR is better as the utility of your spells increases (for instance, on Fizz, CDR is pretty important since his 2 main damage abilities are both mobility ones and you want those up as often as possible) and as your spell cooldowns decrease, since you get access to those "windows of engagement" more easily. On brand, hitting max CDR is pretty desirable as well because at that point you can just spam fireball to stun over and over (~55% stun uptime pre-tenacity).