Disk Question

Ok so in the DS4000 Redbook (page 153) they give a TSM example for setting up disk and they describe using a RAID 10 configuration. What!?! I've always heard and taught that TSM should use TSM mirroring since the extra DB copies are used for writing to help with performance and TSM keeps them all sync'd. So why would I create two RAID 10 arrays and let the DS4000 handle the mirroring? Why not create two RAID 1 arrays and mirror thru TSM? This has implications not only on the DS4000 series but on every high-end enterprise disk out there. So which is it IBM?