Linux does balanced reads on software
mirrors. I'm not sure why you think this can't improve bandwidth. It
does improve streaming bandwidth as long as the platter STR is more than
the bus STR.
... Prove it.
(I have a software RAID1 on this desktop machine)
It's a lot faster than a single disk for random reads when more than 1
thread hits the disk, because it distributes reads to both disks. Thus,
applications start faster, and the machine is more reactive even when the
disk is thrashing. Cron starting a "updatedb" is less painful. It's cool
for desktop use (and of course it's more reliable).
However large reads (dd-style) are just the same speed as 1 drive. I
guess you'd need a humongous readahead in order to read from both disks.
