Thanks Donald,

> Like much of original LaTeX, it is sloppy coding and profligate
> waste of TeX features.

Then, we could say \parshape is here for "historical

> different) for a different list.  The current definition is extremely
> obscure, ugly, and fragile in its redefinition of \par, needed to
> reset \parshape settings every paragraph.

And for this very reason I was expecting an answer
like "well, this way one can do the following
wonderful things...", but it seems the reasons
are quite simpler ;-).