It's possible to lose two drives in the same mirror and lose the array but it's also possible to lose four drives and not lose the array. The most likely time to lose a second or third drive is during a rebuild. With double and triple parity your rebuild time will take forever (I'm talking days) which greatly increases the chances of more failed drives. RAID10 rebuilds are as quick as they get.
If you have proper backups like you said then the performance gain over the rare case of losing two drives in the same mirror is an easy decision for me. Then again that's just me.