Any reason why these would not be a good idea to install on an early MCS which does not have the shield integrated into the factory fan?

 

Provided it doesn't block airflow to the fan, then sure, it's a great mod. Alternatively you could just install the new design shield with an integrated shield.

 

These will stop the larger items from damaging the fan blades themselves, but by far the biggest reason for fan failures is the actual design of the fan. The fan shroud itself allows water and debris to actually fall into the motor area from the backside and jam up/corrode the motor.

Best maintenance for the fan is to periodically check to make sure it actually turns by physically turning it to make sure it isn't clogged with debris. I check mine at each oil change and a couple of time in between each change.