Tim,
Depending on how it is wired, would help to determine why the PO would have it installed. If the permissive switch that allows the OD to only engage in 3rd and 4th gear is bypassed, the PO may have elected to install this to remind him/her that the OD is engaged. This would not create a problem with the permissive circuit working, since it will automatically disengage the OD once you come out of 3rd or 4th gear. However, if your permissive switch is bypassed OD will be could engaged in any selected gear position.
If the permissive switch is wired in was not making up consistantly, maybe the PO wanted an indicator to see that the permissive switch has successfully made contact, allowing the OD to be engaged.
Not sure if this helps....just my thoughts.....Tony