It's not so much a defense against bugs, as it is a defense against the centralization of protocol development (that carries with it the risk of increasing the damage done by bugs).
It may be safer to be on the side of the majority, but the question is should we try to make the majority less major, or is that too risky (something that many people seem to believe)?
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Bitcoin is based on consensus. And consensus by definition needs a majority. So it makes sense that the majority runs the same code.
Since they agree on the rules already anyway. So why use another implementation?
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