Feeding the Trolls
^^^ This.This is my thought too, also if you pay the developer there is a better chance of customer support and quick access to updates/fixes
If you would allow people to download for free, they may just do that to check things out, similar to an elaborate demo.
Then, if they like it, they would prefer to pay if they get continuous customer support and access to regular updates.
I do believe such a strategy might work. The great thing about software is that continuous support and updates are easily distributed. This may be software's most important advantage. So, if you focus on adding content and updates, people would have all the more reasons to pay. Even if you allow them to get the base package for free.