This way, the developers get paid fairly, for what they actually deliver, but they can't lock the users in. If the developer dies, goes bankrupt or falls into a predatory phase where it milks the product for cash without reinvesting sufficient back (Adobe), then the project can be salvaged as open source by those who use it. An even better option is to allow any commercial developer, after the 3 year period passes, the option to fork and develop it's own version with similar licensing conditions.
This will ensure a healthy, competitive environment where the software can be monetized traditionally, yet the 4 freedoms are (eventually) satisfied. As a user, this is what you care about above all else; you will pay the commercial license fee to get the bleeding edge product, knowing you are in a way "leasing to own" the software and you will be able to fork it in-house if the way you use it diverges from what the publisher will want in the future.