My concern with pass phrases like that are the available hacker dictionaries. So while a 20 character simple pass phrase may have more individual character combinations, a hacker program designed to try word combinations instead of every individual random string of characters is theoretically going to be able to crack the password much quicker. With password managers to handle them, it's just as easy and more secure to make that a 20 character string of random characters. And if you are using pass phrases like that, then I'm guessing you are not using a manager, meaning you are manually typing in your passwords. With an auto-fill ability, now the manager is both easy, more secure, and quicker at logging you in.
There really isn't a reason not to use one in today's world.