Defects tend to happen to all the units in a certain production run. If one goes, the others may go before you can replace & rebuild the first bad drive.
Fair enough.
I'd still prefer to have all my RAID drives be the same manufacturer + model. Maybe that's just an ingrained habit I picked up in the 1990s.
Indeed ... I thought it would be better to have all the same, as I remember different brands of drive sometimes had incompatibilities (WD and Maxtor spring to mind). Also, as it defaults to the smallest size, having all drives the same means you don't lose any more space than the parity drive.
Cheers