Widelands is an open-source game, not a life-critical system, neither a customer-paid product. Therefore, the best release-model should be the one that brings the most convenience to everyone involved. In my opinion, that would be to have a minor release every time there is some major bug-fixing or some major feature-addition, meaning something that pushes the game's quality considerably forward.
And who is the one who decides which bug will push the game's quality considerable forward? Every dev works just on the bugs he/she is interested in, thats how open source works.