The 3ds Max SDK embeds a core referencing mechanism (ReferenceMaker/ReferenceTarget) which embeds methods for notifying dependents of changes in the reference hierarchy. This mechanism is very general and powerful, but lacks context to identify and filter relevant messages. It also tends to be error prone: creating references to scene objects, for the sole purpose of watching for changes, may itself result in disrupting the reference hierarchy, creating unwarranted notifications as well as artificially hanging onto objects which should be garbage collected.
Furthermore, certain types of changes do not trigger messages in the scene's reference hierarchy; they require different methods of monitoring which have simply not been available to the plugin developer. For example, changes in a viewport that is not attached to a camera cannot be monitored in any way.