In general the Windows binaries are not really a fixed target for Wine either - many applications (e.g. multiplayer games) have online components that require you to run the latest version. And even for others, people will demand the latest version work. Fixing things that are actually work first is still a good way to prioritize things - it's not like you stop implementing functionality once one target program works, you just move to another one. And if you run out of interesting programs then you can look at 100% API coverage.