If only ALL third party installers worked this way but alas they do not. It is usually with Third Party installers that the “permission” woes occur. Also many programs (suchs as Printer files or iTunes) may have inncorrect settings applied during installation which until repair permissions is run can cause problems because most users do not operate their account under root. Root may used in the installation process but if a file has the inccorect permissions set once the install is over and the user tries to “run” the program and they are not signed in as root then a problem could arise which could lead to corruption of other files (prefs, cache, etc)
Once a program is launched the first time if it makes or attempts to make changes and runs into permission problems or causes permission problems the results can be odd at times or can cause future problems down the road leading to corrupt cache files, preferences, etc which have to be rooted out and repaired. A cascading issue that can arise due to a “faulty” installation and incorrect permissions set for the “User” when they try to launch the program using permission sets not set as root.
As mentioned before the actual INSTALLER does NOT repair permission so a file may be assigned incorrect permissions during the installation period. An example is this. If it were as you say then anytime iTunes is installed it should automatically be fixed of any permission issues. But this is not always the case because during the installation process the wrong permissions sets could be applied and would only be corrected AFTER one ran Repair permissions which would then compare the current Permission Settings that a file has to the default settings in the receipt. If any changes are detected they are “reset” back to the correct values in the receipt. But until you run Repair permissions the incorrect permission settings could still be in use.
A simple test is this. Run repair permissions AFTER a brand new installation of OSX and you will find some permissions had to be repaired. If the “installer” worked as you thought then after an installation your permissions should be already fixed which is not the case. Because a User does NOT run their system while signed in as root and even though an installer may sign in as root sometimes to Install the proper files where they should be placed that doesn’t mean the files are set with the proper permission setttings 100% of the time. Hence once you are done you are running an “unstable” system until you “reset” your permission settings with Repair permissions while under the USER account and not as Root.
Some culprits for hooky permissions settings after an installation are
iTunes
Quicktime
Adobe Products (many)
Microsoft Products(sometimes)
Printer Files and Updates (notorious)
And files dragged from a Locked Disk or “Network Disk” can sometimes have permission woes. Many people see that if they copy a file to a networked server or say a CD-R sometimes they have “read Only” access and have to change the permission set manually because the parent settings were applied to the residing files.
In the end it doesn’t matter what method you use, whether you choose to attempt to be preventive or proactive, in the end if there is a permission problem, eventually you are gonna have to initiate a process to fix it either way. Choosing to do it before (preventive) or after (proactive) is soley up to the users discretion.
Myself though when recommending anything to a user I tend to offer the “safe” way. It may take longer but for the average user who doesn’t understand more so on the technical side and are not too savvy about hunting down problematic systems but I noticed they tend to become frustrated or turned off if an unforeseen problems arises without forewarning, leading to the assumption that the advice they were given caused more harm than good.
Kinda like an English teacher taught me. “First I am going to teach you all the rules. Then I am going to teach you how to break them. But in order to break them you have to know what they are first and you have to know when you can break them and when you shouldn’t.”
What ever way you choose is not a bad way when running OS X because in the end it is an elegant system in the way it is setup.
My favorite thing is when a Windows User asks me “How often do you defrag your HD.”
My answer: “I don’t. It does it itself.”
