While I ultimately agree with you, I think that the safe conservative option should be the default. By the time peoples' systems are large enough for the performance difference to actually matter (if ever), they will have had plenty of time to get sufficiently informed about the trade-offs involved. The people who are upset because they get crappy performance from MySQL when they don't even know about indexing tables are not qualified to decide whether risking data corruption is worth a performance boost.

In other words, it's premature optimization.