While this is true for bigger design issues, I fear it is not true for smaller problems like off-by-one accesses and all the common parsing issues.
Smaller problems require a lot of time, attention and testing to be found. To have a single code base can do wonder for this kind of smaller, but often fatal, problems.
There are also advantages to having independent implementations for such crucial software. A fatal flaw that no one caught in one implementation may not take out the whole system.