[gruez]

Wouldn't having a handful of alternate implementations running suffice in making "consensus more robust"? If there's some bug in core, all it takes is one alternate implementation to catch it.

At a more practical level, there's a strong incentive to go along with whatever bugs are present in core, either as a miner or a user (ie. someone making transactions). If there's a consensus disagreement and you're in the minority, then it's basically becomes a hardfork and your side would have a fraction of the hashrate. This isn't too bad if you're just transacting, but if you're a miner this is catastrophic because all your blocks are wasted, because your side of the fork has no chance of being the longest chain, due it it having minority of the hash power.