copies=n was not intended to be used in place of mirroring - it was really intended for systems where mirroring wasn't an option. While the initial proposal didn't call it out explicitly, those that were "in the know" on the thread called this out as a feature to help with failures on laptops. For instance, this message on Sept 12, 2006:

  Dick Davies wrote:
  > The only real use I'd see would be for redundant copies
  > on a single disk, but then why wouldn't I just add a disk?

  Some systems have physical space for only a single drive - think most
  laptops!

  --
  Darren J Moffat

It seems this thread has disappeared from the internet. If anyone is interested in zfs-discuss@opensolaris.org archives, I can probably convince gmail to turn it into an mbox and post it somewhere.

Edit: format. Sorry mobile users, I really need a block quote here.