I have encountered a GeoTIFF raster files with EPSG:4326 (WGS84), i.e. no projection defined! Is this normal? I would expect raster to be in Projected Coordinate System. Raster is a grid of pixels so I expected this would be defined in the plane.
The geotiff you describe does have a projection - it's called Plate Carre. As noted in the link this is neither equal area nor conformal, so it's usefulness is very limited. On the other hand, it is very easy to produce, since the lat/lon of a pixel is trivially derived from the pixel indices.
As to your question "is this normal?", I'm not sure how to answer that. It's certainly well-defined and conforms to the geotiff specification. I'm not sure I've personally ever seen a geotiff defined this way, but that's possibly because of the nature of the geotiffs I work with. If your question really is how common is this usage, perhaps someone else can provide an answer.