Not sure what nginx is like, but in my experience, the developer/operator experience of commercial software tends to be subpar. For instance, when I worked at a shop that used a ton of Red Hat software (millions of $$ per year in licensing), the commercially-supported versions often were a pain, with requirements like phone-home (that didn't play well with the mandatory corporate proxy), documentation behind a paywall and hard to discover (yes, we had login accounts, but Google couldn't index it), and other disadvantages. The OSS equivalents were easier to access, had better (or at least better-indexed) documentation, and we didn't need to worry about per-seat licensing (again, we were paying for it, but we still had to track it).

If you're going to sell software that has an OSS variant, make sure the commercial experience actually outshines the free one.