If developing to standard gives you the same result on each engine then the number of them does not matter.
What if I develop to standard but only test in Webkit. My page is broken in Gecko though. Whose fault is that, Webkit's, Gecko's or mine? If it's mine then why? I did everything according to standards.
So in reality you don't develop against standards (alas), you develop against their implementations, and having multiple engines helps in no way.
In reality, most web developers have no clue about what the standard says in edge cases, which is just fine: neither do most other people.
But the result is that it's very rare to find cases that are really developed "to standard".
Even worse, though, on mobile right now people aren't even trying to develop to standards. UA sniffing and locking out of non-WebKit UAs and use of -webkit-prefixed things even when standard alternatives are available is rampant and purposeful. And when you ask these people to develop to standards they just laugh at you.
In reality you develop against multiple implementations that attempt to adhere to the same standard, where they break in between is where the standard or the implementations need fixed, differences in assumptions agreed and ambiguities clarified. That was the point of the blog post?
So in reality you don't develop against standards (alas), you develop against their implementations, and having multiple engines helps in no way.