Or you buy it for lynchpin fights and endgame when you have nothing else to do with your gold, it's a gigantic power spike for only 250 gold and if you're already at 40% cdr you're wasting stats
On top of inventory space (pink, green, pots, boots, other items) plus cost, this is just unreliable at best.
The only time I opt for 40% CDR through items is when I'm doing ionian's boots + 20% CDR item to hit 40% with 1 item + boots for a huge powerspike early game. Its so critical for skirmishes that your ult comes back 20+ seconds sooner than normal, especially for supports and their game changing ults. 20 minutes into a game and being able to ult every minute makes roaming that much more deadly from you.
I mean yeah, that's most optimal. There are some non-mages that just want 40% cdr though, Maokai comes to mind. Frozen Heart + Locket... Where's your last 10% coming from? Runes/masteries, please... that or wait another 3k gold on Visage, which isn't bad but that's 5 items into your build without Thornmail, GA, or Warmogs. Riven gets brut and ionian, where's your last 10%? etc etc
That said I saw Faker's TF do a thing earlier where he got early Ionian and a blue pot and just started ulting things. Was super legit, should try that **** out.