I would say not all air cooled bikes need them, but many would benefit from them, especially V twins due to the lack of airflow over the rear cylinder. Some, like an old Ironhead sporty do indeed almost need them if you want them to live a decent while in stop and go traffic.
But others like the old BMW opposed twins don't particularly need them.
I dont think you can go wrong with an oil cooler and with a Harley you can even buy theirs at the same cost as aftermarket.
I have been checking my oil temps on a daily basis and contrary to popular belief I have found, to my own surprise that the faster you go, the hotter your oil gets (unless of course your dead stopped in traffic but actually, I dont even think in traffic I saw them as high as = read below)
I was surprised but think I now know why.
At high speeds your burning much more fuel, generating much more horsepower, much more resistance such as wind resistance. There is much, much more load on the engine at high speeds cause oil temps to rise.
I have found, at interstate speeds, on my 14 Road King with factory oil cooler, day time temps of 90+ degrees, constant 75 to 80 MPH for 30 minutes or more that my oil temps are always the highest then any other speed, normally around 248 to 254 degrees. The highest i ever measured was 264 degrees but cant remember if that was traffic or interstate.
I suspect this is why the oil cooler in the first place and why manufacturers do not put fans on them, they know its higher speeds at MUCH greater loads that generate the high oil temps.
At slower speeds I easily attain (average) what Harley calls normal temps of 230 degrees. Around town less then that, a mix, right around that and long trips on the interstate much higher then that.
Its really quite interesting and caught me by surprise but if you think about it, makes a lot of sense. At high speeds, more fuel, more horsepower being generated and much, much more resistance that the motor is working against, namely wind.
Since I live in such a hot climate and travel at high speeds so often, I know when I get around to it, Im pretty sure I am going to upgrade the oil cooler from the stock 6 row to a 10 row as hands down, no more question, the high speeds really heat the oil up. The engine is designed to handle this, but what the heck, this is my hobby too, so would like to throw a 10 row on it and see if it stays under 250 in high stress, high speeds as a fun project.
Its pretty much plug and play for a 10 row, take off the 6 row, put on the 10 row.
Wards fans on their website has the stats on the VTWINS and surprisingly, the rear cylinder temps arent drastically higher then the front, yes, statistically significant, but 10% or less which isnt all that much.