I'd guess that writing to the RAID is still bottlenecked by the worst drive in the array. But if only one out of N blocks has to be read/written to that drive, average performance may improve (basically, the idea of 'regression towards the mean').

I don't know if that's what actually happening, but testing seems to confirm that RAID performance is more consistent than single drive performance.

RAID0 on EC2 isn't as insane as it appears at first glance. EBS's have an annual failure rate of around 0.2%, while hard drives are around 5%. So, the chances of 1 EBS drive failing is around the same as the chance of 2 physical hard drives failing (5%^2 = 0.25%). It doesn't sound unreasonable to suggest that a RAID0 of EBSs is about as reliable as a RAID10 of physical drives and more reliable than a RAID5 of physical drives.