If you look at fan curves for any cooling fan, they are all about air flow at a specific RPM at a given pressure drop. Raising that pressure drop with a plugged core really does change the fan performance. On a lot of the newest mobile equipment where they run a hydraulically driven fan, they actually use a reversing valve to run the fan backwards on startup and/or occasionally to try to blow debris off of the front face of the cooler. Some designs utilize a square wave on the external fins to make for a lower clogging core, but they do not have the same cooling as the squiggly or perforated fin designs, which by the way are considered to be more prone to clogging. Bottom line is there's no free lunches: the higher clogging fin designs perform better when they are clean, but they are harder to keep clean.
Along with several other earlier comments, I believe a lot of suspect fan clutches probably are victims of plugged radiators. I ran my old IDI without a fan at all for several years, but my cooling system was pretty new and the radiator was super clean.