Why is 'map' used in filter-branch's skip_commit?

Hi,

I was wondering why skip_commit uses 'map' to rewrite the parents
supplied to the commit-filter.

For a moment, I was worried that if skip_commit has to rewrite parents
then the user should have to, too, before passing them to git
commit-tree - but no, it seems that the parents supplied to the
commit-filter are already rewritten.  In fact, before skip_commit was
introduced (commit f95eef), an example in the filter-branch
documentation used 'echo' instead of 'map' for the same construct.

Is it just a matter of style?  Or am I missing something else?

Thanks,

Jonathan