Description of the Journaling topology

[Martin P. Hellwig]

Matthew Dillon wrote:
<cut see op>
    That's the concept in a nutshell.  In addition to all of that the data
    being journaled will have a number of options... e.g. the journaling
    data stream could be a simple non-reversable stream (a 'replay' stream),
    or a fully reversable stream (the ability to 'move' the regenerated 
    filesystem forwards or backwards in time simply by playing the journaling
    stream forwards or backwards), etc etc.  It is going to be a *VERY*
    powerful mechanism that no other BSD (or even Linux) will have.
<cut>

Barely understanding the implication of this concept it strikes me 
mostly logical, clean and relative simple.
Which makes me curious why other project haven't done this already?
What is the major reason that other project follow a different path then 
this one?

--
mph