In this case, I think the system perfectly justifies having 50m homes empty. If you look at it another way, if San Francisco (arguably another booming localized economy) had the same initiative to build a 20% buffer for housing, it would be seen as a progressive, citizen minded effort to create affordable housing and a huge benefit to the community.
As China, I'd much rather face the problem of building empty homes (which is not really a problem?) than face the population revolting from huge spikes in cost of living due to not enough urban housing. SF is full of rich enough people to ignore the struggles of the working class - China probably isn't.
edit: I also don't think the rise in 2nd or 3rd home purchases is an issue, it's more of a symptom of the growing divide between rich and poor, especially in China.