True. However, many exceptions
If you are visiting, you're going to use google maps while either in a cab or on the subway much more than you think. And that app takes way too much data.
I don't think it's necessary
によるポスト Kyle Sterret に 韓国 フォーラム
You know you can just use the internet at your hotel and coffee shops right?
I experienced same situation
Incheon airport has a few counters you can pick one up at however I strongly recommend you reserve one online. Widemobile all the way. Cheapest unlimited data unit available. However they rent theirs out super fast so there is a chance they don't have any available. There are other options like KT and Olleh but they are a bit more expensive for unlimited data.