Currently m2 pom require to have explicit version of the parent pom in all submodules. This should work fine for "standalone" artifacts. However there is different type of projects (e.g. EAR) that is usually stored in version control as a whole thing and may contain multiple modules (ejb jars, rar, war, etc) that are build with the same version as entire EAR. So, EAR application released with a single global version for all submodules. In m1 this was possible to specify relative path to parent pom and use version substitution in child modules and deployer goal was substituting values for version numbers upon deployment.
For this kind of modules it is very important to have single palce that will have global version number, which then used for submodules. It is also make sense to kee optional relative path (that can be also removed/replaced with concrete parent id/version upon deployment) to support local build. I believe this is very important for J2EE builds as well as any other projects that are releasing multiple artifacts/jars under the same version (e.g. ASM, XDoclet).