It was a 20-something disk RAID 10 [1], arranged so that every mirrored pair of disks spanned different enclosures, in order to mitigate the failure of any one shelf — that is, interleaving mirrors across controllers and shelves, exactly as you suggest I should have done — and further, such that any one shelf failing only affects the mirrors that had disks on that shelf.
EMC's software wanted to allocate the drives from two shelves, with an unequal number of drives per shelf. It was just grabbing the next however many disks, linearly.
So, no, they weren't trying to balance the mirror across enclosures or controllers. They just weren't thinking.
[1] By "RAID 10", I mean "striped mirrors" — that is, create a bunch of mirrors that span shelves and then stripe across them — not "mirrored stripes" which is what you appear to be suggesting, with "it is better to have two disks on the same controller/chassis in raid 0, then raid 1".
A striped mirror is recommended in everything I've ever read on the subject, because it puts the redundancy at the lowest level of the array's geometry.
Using a mirrored stripe, on the other hand, means that when one disk fails, any other disks striped with it, still presumably perfectly functional, can't be used; the controller must instead read from and write to the mirror. If a disk in that mirror subsequently fails, you've lost data — and remember that when striping, the chance of failure is multiplied by the number of disks in the stripe.
EDIT: Footnote.
