And I guess it would be even more important in a plane where someone shooting at you could easily cause a problem at a very inopportune time.
A very good point not lost on all. I was on the team working on the US101 bidding on the USAF replacement for their Pavehawk rescue helicopters. The US101 is a variant of the EH101 Merlin.
It has three engines. The RN uses these and has been known to fly on only one engine for long range patrol to save fuel. When flying a rescue helicopter into a combat situation, knowing that you have spare engines makes the pucker factor slightly less.
For many years, the USN have single engine aircraft, A-7, F-8, A-4 to name a few. One of the reasons these were retired was the fact they only have a single engine. Of course now, they have bought the F-35 which also has a single engine. The powers to be believe that it is so reliable, that the single engine will not fail. They have once more forgot about combat I think. They are counting on superior maneuverability, speed and firepower to prevent being hit. I disagree with that myself.
Another interesting bit of history is the US Army. Back in the 60s, they needed an attack helicopter. One of the contenders was the AH-3 (S-67) Blackhawk.
This helo was fully acrobatic from what I read and could carry an arsenal of munitions. It did not win though, the AH-1 Cobra did. The Army cited the main reason was because the Cobra had one engine and thus is more suited for field maintenance than the two-engine Blackhawk.