For example pom.build.outputDirectory (classes directory) should inherit from pom.build.directory. Same for all properties that are used to generate build data.
Why? Becasue otherwise it's downright impossible to change the location of the target directory (pom.build.directory) as you don't know beforehand all the plugins that are part of the lifecycle and how many properties they each have and their name.
For example in the clover plugin I need to set the target dir to another location so that main build data is not compromised. I can't do it as I can't controll all the properties that could possibly exist. I'd just like to set the pom.build.directory to a new value and be done with it.