If you want to get software updates (and you /do/ want it, even if you don't know, because of critical security fixes), you reach the point of needing to change the device software in only a few years (see, even Google's own Nexus line has a laughable software support duration: https://support.google.com/nexus/answer/4457705#nexus_device...). In the lower end, it frequently happens before the end of the legal warranty in some countries (eg. 2 years in France), especially with discontinued devices sold during sales. Worse, there's a frightening number of phones sold on Amazon that run an /already/ unsupported version of Android with known security issues.

Then, when you have the chance to have a device supported by some third party, you discover that you can't do backups without having to appeal to some wizardry with ADB and your phone in “developer mode” (when you even have that option), because you can't otherwise do proper backups without having rooted your phone /before/ having data to back up in the first place (ie. thus voiding your warranty from day one).