Anyway, my point was that if we have enough competing programs, they are forced to be compatible with one another. Right now most people care only about Adobe Reader, but people in the free software world perhaps care only about poppler-based readers -- hence bugs on both sides.
Another example is HTML -- once upon a time, there was only one dominant browser, Netscape; then only another dominant browser, IE. There was an open standard and a standards body but the de facto standard was whatever the big guy did. That has changed thanks to competing browsers, especially on mobile platforms. The fact that two of the engines (webkit and gecko) are open source seems, to me, irrelevant, except maybe to the extent that open source developers are more mindful of standards -- but many would argue that Opera is more standards-compliant than Gecko, while webkit, though open-source, has been driven primarily by one company with a very closed-source mindset. The important thing is that there is competition and therefore interoperability is needed. It's the same thing that ensures plug points, screw pitches, pedal layout on cars, etc, are reasonably standardised.