The problem is to tap all of that into your phone, every time iOS decides it desperately needs it again, with just stars instead of letters. That's annoying.
I've got a fairly long iCloud password with upper and lower case letters, numbers, and symbols. After four or so years of typing it on iOS devices, I have the muscle memory to type is extremely fast without thinking about it. In fact, to figure out the actual characters of the password, I have to visually reconstruct the physical typing that I do from muscle memory.
It's a pain, but really not that bad. You tweet from your phone (or use email/SMS/whatever else). 20 characters is manageable and secure, as long as it's randomly generated.
