Note: see the discussion in the outdated diff above. 

Including this parameter is the only way a reporting system can get an activity type or group of activity types without cloning the full list of statements in the LRS. If a reporting system has to clone the LRS, then what's the point of the LRS, why not just send statements directly to the reporting tool, and why do we have the other filters like agent, verb, object and instructor?

If we can't filter by activity type, then what is the purpose of activity type in the statement? 

From my view, this is more important than instructor and activity and on a par with verb and agent.