It's really not so simple. You cannot claim: "by having N competing but mostly identical projects a bug can only be used to take down 1/Nth of the computers". It is more like: "by having N competing but mostly identical projects you increase the number of bugs in your system N times".
And if we had 10 competing implementations than we'd have 10x more bugs. That sounds pretty bad to me.
I think nourishing this kind of competition is a bad tool to combat computer insecurity. If you have a single well reviewed implemention I think you are much better off than having 10 badly reviewed ones.