And not to forget the shield provides splash protection when driving in rainy conditions or through puddles. I personally would put it back on. Otherwise water splashing from the wheels will really splash very deeply up into the engine bay.

 

Believe the power steering hose is also protected by the shield
Keeps things cleaner as well as the electronics drier

 

+1 on keeping undershield in place. Besides. when you need to troubleshoot a leak, it does help when engine is not splashed all over.