Example: MS actually stops to allow to distribute their MSDE as "includable packages" (sorry, don't remeber how it names exactly), when you could include some parts of the MSDE's installer into your one. The reason is very easy: you have to provide than the critical updates for MSDE (issued by MS) yourself, through the your installer/updater. If MSDE is installed with MS installer, than these fixes are not your problem, but MS (I mean, how to distribute and install them).
P.s.: you can be a really nice guy and send e-mails to your customer about things released by MS.
Most of the time these "parallel" libs in /lib are simply the links. This thing is really annoying me. Because of that personally I prefer only Gentoo. The most perfect "package manager" for Linux. But the slowest in adding of the new software on your PC