Counter Argument: (Not saying I disagree, just a different point of view)

It is possible that having a commercial offering, or a way to actually make money off nginx, will simply increase development time over all. Yes, some (most) of that will be towards the commercial product, but if they are making a reasonable amount of money off this, it may mean more time spent on the open version as well.

Also, with the nature of git it is possible for someone, or a group of someones, to fork nginx and run that fork as the 'new' open source nginx. What I mean is someone could fork the repo and if that one is getting enough attention from open source devs, it is possible that the fork would become the recognized version of nginx in the future. Not saying this is overly likely to happen, but it is possible.