imo, You don't want to have all three parks closed off for special events at a time when they are likely to have well over 10,000 on-site room, possibly more towards 15-20,000 once the South plot of land and Wet n Wild plots are built out.
There needs to be a park for these people to go to just as a regular guest. They certainly all won't be able to fit into HHN on a given night (and many won't even want to) so a MNSSHP equivalent is a natural addition.
However, there will be people that just want to go to the park too, just like people want to go to a park when MK has MNSSHP or MVMCP. During those times, people still have 3 other parks to choose from. My point is, there needs to be an alternate option besides upcharge events.