Using the source for the install standard allows all distros to have a common basis from which to work from. It is pretty straight forward to identify dependencies and build options.. and to build scripts and installers for specific distros.
The original ACS docs used RedHat as a basis. Now, RedHat has.. RedHat Enterprise and Fedora.. The docs would be less useful to Ubuntu and FreeBSD and Windows users, for example, if they just instructed to use RPMs --not to mention that RPMs were not always available for some software required by a recent OpenACS release --libxml2 and the latest stable tcl release(s) for example.
The immediate convenience and usefulness of any specific install script is limited to the extent that it actually works without errors in a particular distro. If one happens to be using a different distro, one would have to likely learn the idiosyncrasies of 2 distros for the script to be useful --and that can require learning the idiosyncrasies of a 3rd *nix to figure out where the "norm" is.
Since Malte's script is mentioned, let's consider look at how that might break on something other than a Debian install. 1) It assumes one has installed Aolserver4.5. What if the user wants to use nsopenssl? Currently, it is not clear if nsopenssl will work with 4.5. 2) the script depends on wget. Wget is not directly available on many distros. One has to download it or know the distro's alternate (and how wget works by default since no flags are shown). 3) the script uses "make". Some distros invoke gcc with "gmake" instead of "make". 4) Did I mention that bash is not necessarily the standard shell? 5) Daemontools is problematic in certain distros or environments also. Anyway, a simple install script for 1 distro is a headache for another. Besides using an automated installer with it's own build process, one concievably could use tcl to build custom scripts that work under various distros, since tcl is fairly platform independent for our cases, but that's another install method. Anyway, to keep the docs simple, a standard was created that has value regardless of the specific distro.