I think this is intended behaviour
Oh please not. What would be the rationale for that? If a user explicitly declares a <version> element under <build>.<plugins> isn't that a clear indication of this intention to use that version? Do you want to explain to users why that is ignored and they need to hassle with setting up a <pluginManagement> entry just for the fun of overriding a plugin version inherited from the parent? Do you want to clarify why the little sentence "Plugin Management contains plugin elements in much the same way, except that rather than configuring plugin information for this particular project build, it is intended to configure project builds that inherit from this one." that users learned from http://maven.apache.org/pom.html#Plugin_Management is not quite up with the reality?
Last but not least, the lack to override the version only manifest itself
a) for a lifecycle phase, not for a direct mojo invocation
b) only for the first lifecycle phase
what if you change the pluginManagement of child modules?
As I already commented in the attached demo POMs, this successfully allows the childs to change the version.