The cooling system in good condition has the capacity to sit in traffic at 115F with the A/C on or climb Sunset point (6 degree 7 miles) at 65 MPH and 100F+ without overheating, so it has plenty of reserve at 75F. The fan will block some flow when it's not running and add flow when running, a design used in many cars, including foreign spec 80's. When the A/C is working it removes heat from the interior and dumps it in front of the radiator, adding greatly to the cooling load.
In hooking it to the compressor it helps the cooling system at the highest load times and improves A/C performance at low speeds. Here in Arizona we run the A/C most of the year, a lot of times including on the trail, so it covers most of our needs. One option that it doesn't cover in this mode is the emergency overheating, where you want the fan running but don't want the A/C on. The A/C comes in the winter to dry the defrost air and the fan is not needed/wanted in that mode. Looking into using a rear heater fan switch with it's on-off-on to be able to turn it off when not needed and have two modes.
Some of the ways to wire the fan;
Running when the ignition is on; Advantages, none. Disadvantages, it runs when not needed putting unnecessary load on the charging system and wear on the fan, slows warm-up in the winter.
Manual switch control; Advantages, driver can run it any time it's needed. Disadvantages, driver has to remember to use it and know/anticipate when it's needed.
Ignition on power, cooling system thermostat controlled; Advantages, runs when the cooling system needs it. Disadvantages, Isn't good at anticipating A/C loads, Doesn't take advantage of after shutoff cooling. This is basically how the fan clutch works, so on our trucks it's just doubling up on one strategy, not taking advantage of others.
Always on power, cooling system thermostat controlled; Advantages, runs when the cooling system needs it and continues after the rig is shut off to help with heat soak. Disadvantages, Isn't good at anticipating A/C loads. This is how most manufactures control the main fan on an electric fan only system.
A/C controlled; Advantages, anticipates A/C loads, somewhat driver controlled, when I am hot and the A/C is turned on, the truck is probably hot too. Great at lowering idle/slow speed A/C vent temps. Disadvantages, Doesn't take advantage of after shutoff cooling. It's not connected to the cooling system, so the truck could overheat and the fan never come on. This is how most manufactures control a secondary fan, either by tapping the compressor clutch or a high side pressure and/or temp switch.
All strategies have compromise, the A/C controlled works the best for us. The advantages for us is that it's easy to wire, no switches or underdash work, nice and simple. The disadvantage is some lack of control, but they are easy to overcome, to turn it off in the winter simply pull the fuse and the Bosch relay we used has two output connectors, so in case of emergency overheating just open the hood and move the power wire to the second output and it's always on.
Sorry for the looonnngg winded reply!
Some thoughts on ways to trigger it in this post. 
