I believe my netbook puts /tmp into RAM, which means that the data in there is completely lost if the machine reboots. That's trading performance for a complete lack of reliabile storage over system restarts.
I think you'll find that people's needs vary from 'data absolutely must get to the disk and stay there' (e.g. online sales order database) to 'I want performance and if I lose the file on a reboot I don't care' (e.g. said temporary files). Different filesystems serve different needs.
That said, I agree that a filesystem for general uses (e.g. extN) should put reliability above performance.