Adopting more “permissive” licences may well encourage coders from the proprietary world to join in; but the price of them doing so is that the fruits of the community's endeavours can be taken and turned into closed source. That's problematic for two reasons.
The fundamental problem is that efficiency in this case results in less freedom: closed source software, by definition, does not respect the freedom of its users, so the more of it that exists and is used, the greater the lack of respect for that freedom. It's like putting golden handcuffs on someone and emphasising how valuable they are. But even at a more pragmatic level, this approach has its disadvantages.
The success of free software is based on the insight, at once profound and yet trivial, that it is much more efficient if everyone contributes a small part to a greater whole that they can build on and benefit from, than for everyone to toil away on their own, constantly re-inventing the wheel.
It's why free software is able to grow and mature so quickly. Whether people do this out of pure altruism, or because they recognise that it is ultimately in their own interests, is not really the issue: the system works. But it only works provided there are no free riders.
Research show that people in general are willing to contribute to communal projects if everyone does so on the same basis. But as soon as it becomes possible for others to take without giving – for free-riding to exist – there seems to be some primitive instinct that revolts. Indeed, many studies show that people will actually go out of their way to punish free-riders, so deep is this innate feeling of injustice.