If the oiler was removed (and possibly thrown away) is it an ABSOLUTE necessity to have or can I get away without one?

The oiler allows for expansion of the oil in the drive.
It has to go some where. Pressure builds up in the drive and you will blow a seal. That's why it is there.

Ya, and you don't want to blow a seal!
http://www.youtube.com/watch?v=R9ETlTZoF1E

There are some nice ones aftermarket. Just make sure they have the proper fittings for your level sensor. Get larger ones because the small ones on mine overflow all the time if the fluid is anywhere near full.