many programs are using libpng-1.6.x when we are compile with libpng but something is yet needed to use libpng-1.5.x such as the wine (there is currently no compilation problem but malfunction is being when we build wine without libpng-1.5.15. I've filed this bug into the winehq upstream.). AFAIK there is no special slot for the libpng-1.5.x (it just having number 0 as default) but libpng-1.6.x has a slot number 16. so I think that we need additional slot for the libpng-1.5.x for some packages' successful build.
If there are any packages in the tree that don't build against libpng 1.6, a bug needs to be filed against each specific package, now that libpng 1.6 is marked stable. Unless there was already one, that was fixed and it's just a matter of marking a more recent version stable. For libpng slots other than 0 are meant for binary-only packages. (as for that wine bug, you've mentioned, I strongly suspect it's invalid too)
*** This bug has been marked as a duplicate of bug 499952 ***