Algebraic manipulation of patches vs. dependencies

I've been trying out Darcs recently, finding it useful, and trying to
understand how it works.

What puzzles me is that Darcs does not store a dependency graph of all
the patches in a repository.  Instead it keeps patches in a list, and
manipulates the list algebraically, reordering patches that commute.
When two patches modifying the same file are reordered, the line
numbers they refer to must be rewritten.  In some cases, Darcs groups
together patches that can be applied in parallel.

Is there any reason why Darcs doesn't go a step further by storing a
full dependency tree?  This would have the advantage of being a
canonical representation.  Patches' line numbers would not need to be
rewritten according to where the patch appears in the sequence.  The
line numbers of a patch could be given relative to the file contents
defined by all the patches it depends on.

Mark