Why would anyone choose this?
It must be for the sake of giving the downstream users some sort of assurance that a Bad Thing won't happen to them, as a sort of promise.
"If we ever commercialize this, we will give all of you the opportunity to do the same."
It seems that the only authors for whom it would make sense to choosing this license would be authors who have no intention of going mixed proprietary licensing now, or in the future. Moreover, authors who believe in copyleft FOSS licenses and want to keep the project that way.
Someone who chooses that license knowing they will likely issue proprietary versions might as well skip straight to a simpler BSD-style license that is implied in that action.
Someone who doesn't believe in copyleft FOSS would also just skip this sort of thing and give the users the "hyper-permissive" license.
No matter what promises a copyright holder makes, they can retract them. However, at least this will be in force for users who hold existing copies. So that is to say, if the authors decide to commercialize and switch to Affero GPL, then only the new issues of the software going forward will be bound by that AGPL; the existing users will have the "hyper-permissive" license for the code up to that point.