I can tell you from my experience that in normal driving highway and suburbs a 300 Tdi doesn't need a fan or fan shroud even in the summer. However, if you get stuck in traffic, even in cool weather, you need a fan shroud. I drove my 110 around for a year with no fan or fan shroud in Richmond, Va where temps regularly hit 100F in the summer. I had an electric fan, but I left it unplugged because it wasn't switched. I had to plug it in one time in October when sitting in a huge line of traffic at an apple orchard.
A V8, on the other hand, will start to run hot a low speeds almost immediately with no shroud. After an exploding fan took out my shroud I cut a RRC fan shroud to fit my V8 D90 and it still would run hot if I wasn't going over 40 mph.
A fan shroud is essential for operation of the fan. If you don't have a shroud, the fan won't pull air through the radiator.