With mining-diversity=0.5 any two miners will be sufficient to continue mining the blockchain. However with mining-turnover=0.0, if one of the miners of the last two blocks suddenly becomes inactive, the third miner will wait longer before stepping in, and the next block delay could be up to double the usual time.
If you're not seeing this behavior on the chain, please let us know, because we'd like to understand and resolve the problem.