As we know from MySQL AB, the theoretical advantage is more investment in the software. That's particularly hard to get early on, with certain kinds of software (not every piece is interesting enough to geeks in general, or has customers with the right skills, to get a community working on it straight away).
In the MySQL AB case they tended to make money from selling licenses for using the database inside proprietary software. This strikes me as a smart compromise. It's something that had it been BSD licensed would have happened anyway, but MySQL AB got money for it that they could pump into improving the MySQL fully open source software for everyone.
Of course, now Oracle own it. But we are no worse off because of that than if MySQL AB hadn't retained ownership - we can just ignore Oracle's ownership, fork it, and turn it into a more standard project with multiple copyright ownership.
I don't think *every* piece of open source software needs this business model, but there are some projects which do benefit from copyright assignment in this way. Diversity is good!