If someone should ever need this, here is a more solid version of the workaroung:

      #region mainGrid_SelectedItemsChanging

      //workaround!!!
      private void mainGrid_SelectedItemsChanging(object sender, Infragistics.Windows.DataPresenter.Events.SelectedItemsChangingEventArgs e)
      {
          //Unselect all previous selected DataPoints
          ManageDataPointSelection(mainGrid, mainGrid.SelectedItems, false);

          //Select all new Datapoints
          ManageDataPointSelection(mainGrid, e.NewSelection, true);
      }


      #endregion //mainGrid_SelectedItemsChanging


      #region ManageDataPointSelection

      /// <summary>
      /// Manages the selection of the DataPoints.
      /// </summary>
      /// <param name="grid">the grid</param>
      /// <param name="selectedItems">The collection of selectedItems of the grid</param>
      /// <param name="select">Determines wheter to select (true) or unselect (false) the dataPoints</param>
      public void ManageDataPointSelection(XamDataGrid grid, DataPresenterBase.SelectedItemHolder selectedItems, bool select)
      {
          Model.Indicator indicator = null;
          Model.DataPoint dataPoint = null;

          //SelectedCells
          if (selectedItems.Cells.Count > 0)
          {
              foreach (Cell cell in selectedItems.Cells)
              {
                  indicator = (Model.Indicator)cell.Record.DataItem;
                  dataPoint = (Model.DataPoint)indicator[cell.Field.Label.ToString()];

                  if (dataPoint != null)
                      dataPoint.IsSelected = select;
              }
          }

          //SelectedRecords
          else if (selectedItems.Records.Count > 0)
          {
              foreach (DataRecord record in selectedItems.Records)
              {
                  indicator = (Model.Indicator)record.DataItem;
                  if (!select)
                      indicator.clearSelectedDataPoints();
                  else
                  {
                      indicator.SelectDataPoints();
                  }
              }
          }

          //SelectedFields
          else if (selectedItems.Fields.Count > 0)
          {
              foreach (DataRecord record in grid.Records)
              {
                  foreach (Field field in selectedItems.Fields)
                  {
                      indicator = (Model.Indicator)record.DataItem;
                      dataPoint = (Model.DataPoint)indicator[field.Label.ToString()];

                      if (dataPoint != null)
                          dataPoint.IsSelected = select;
                  }
              }
          }
      }

      #endregion //ManageDataPointSelection

After looking into this and doing some research, AddedItems and the RemovedItems Properties of the SelectionChangedEventArgs class has been determined to be a new product idea. You can suggest new product ideas for future versions (or vote for existing ones) at http://ideas.infragistics.com.

There are many benefits to submitting an product idea:

-          Direct communication with our product management team regarding your product idea.

-          Notifications whenever new information regarding your idea becomes available.

-          Ability to vote on your favorite product ideas to let us know which ones are the most important to you.  You will have ten votes for this and can change which ideas you are voting for at any time.

-          Allow you to shape the future of our products by requesting new controls and products altogether.

-          You and other developers can discuss existing product ideas with members of our Product Management team.

Steps to create your idea:               

1. Log into the Infragistics Product Idea site at http://ideas.infragistics.com (creating a new login if needed).
2. Navigate to the product / platform channel of your choice (e.g. WPF, Windows Forms, ASP.NET, HTML5 / Ignite UI, iOS / NucliOS, etc.)
3. Add your product idea and be sure to be specific and provide as much detail as possible. Explain the context in which a feature would be used, why it is needed, why it can’t be accomplished today, and who would benefit from it. You can even add screenshots to build a stronger case. Remember that for your suggestion to be successful, you need other members of the community to vote for it. Be convincing!

The Product Idea site puts you in the driver’s seat and allows you to track the progress of your ideas at any time, see how many votes it got, read comments from other developers in the community, and see if someone from the product team has additional questions for you.

Thank you for contacting Infragistics.

Thanks for the info. So do you think this will be available in the next version 13.2? According to the release dates that should be Nov 10, 2013.