From looking at the code, we use `_billboardsToUpdateIndex` to avoid having to ever do `this._billboardsToUpdate = [];` (since that causes memory churn ).  The only time we change the size of `_billboardsToUpdate` is when the total number of billboards overall shrinks (see the if block on Line [1199](https://github.com/AnalyticalGraphicsInc/cesium/blob/issue1691/Source/Scene/BillboardCollection.js#L1199)) and on removeAll.  Everywhere we iterate we use `_billboardsToUpdateIndex` and not `_billboardsToUpdate.length`  So I'm not sure why we would want to make the change you suggested on line 1117.  Of course I realize you wrote this code to begin with, so if I'm missing something let me know and I can make the suggested change.