On 23 Nov 2009 at 16:59, Sridhar Ayengar wrote:
  Lots of slots.  If the system board goes bad, you can
replace just the
 system board instead of taking the whole machine apart to replace the
 backplane. 
...and many of the commercial passive backplanes are larger than the
usual 8 slots or so.  11 and 13 slot ISA backplanes come to mind.
--Chuck