HOW IMPORTANT IS A FAN CLUTCH,CAN THE FAN JUST STAY RUNNING,

The major purpose of a fan clutch is to save fuel economy and horsepower. If the fan was locked to the engine speed, it would rob power and would be very noisey when accellerating. Most fan clutches are thermostatically controlled, varying the lockup according to the engine temperature.

I absolutely agree with b-glaste's answer. I use to remove the thermo clutch fan assembly from my trucks and modify them with a high output, temperature controlled electrical fan for performance / fuel economy issues, however it cost more then a fan clutch replacement. Zee

Yes your fan can run constantly, but you are waisting gas. A fan Clutch was designed to only kick in when the engine heats up, therefore cooling the engine off when needed. After it cools the engine down, it kicks out, saving a load on the engine, and your billfold It is your best interest to replace a cluth fan with a "new cluth fan" and not the cheap fan, that has no clutch....