The main reason to use an AIO is space limitations or looks.
If you dont like the look of a big tower cooler, then an AIO can do instead, or, if you're in a case like my rig, there's just no room for a traditional air cooler.
Otherwise, i'd stick with something like a Noctua NH-D15 or BeQuiet! Dark Rock Pro. They cool just as well, are cheaper, and have less points of failure.