Well you actually very probably do need PostGIS if you're doing real spatial analysis.

PostgreSQL geometry field are just that, 2D geometries, on a plane, in no particular projection system.

Spatial geometries are relative to a projection system. Try to "draw" a square with GPS coordinate and project it in your local official projection reference, you'll probably not get a squared.

Now, if you don't need precision, feel free to use geometries. But be very aware of the trade off you're making. The article, in my opinion, dismisses them a little too fast.