Exactly, a prebuilt pc such as an all-in-one can work in an environment when you are on an upgrade schedule anyway, such as a corporation or something. Also, it can work if your software needs never change, like, you don't want to upgrade anything software wise or keep up with trends. So in production environments, this can work. Otherwise, things can be frustrating in terms of wanting to do "cheap" upgrades to a prebuilt machine.
The beginning of wisdom is this: Get wisdom. Though it cost all you have, get understanding. - Proverbs 4:7