Currently mass update is disabled in a grid that is default in edit mode (grid_default_editable=1). Even if it this option would be enabled, it wouldn't be very helpful because with the edit mode on it is only possible to select one row at a time right now.
However, it would be very useful if mass update(+multi select) could work in a grid with default edit mode because both functions are great for the same use cases (a list where it is very easy for a user to edit certain values).
