Since the Raid 5 rebuilding is often done in the background it isn't as
obvious to most users.  Also it lets you build a single volume (as seen by
the e3000), using the space on many drives.  Finally the space is used a
little more efficiently then straight Raid 1 mirroring.  But when reading
from the drive you often must wait for the corresponding track to be read
from many drives before you can re-calculate the parity.  This results in
the array appearing to be some what slower then a single disk and much
slower then reads for an Raid 1 array.

So it becomes an issue of what do you need most,  disk space, or speed.