I've often thought about this. Here's what I understood from various sources:
The grease wasn't supposed to be used (except when it was according to the manuals?) because the the gears and the grease both were petroleum products and could break the gears down.
But old dry nylon / plastic is brittle. Wouldn't grease help prevent that? I've had broken nylon gears to replace and as many or more were dry than were greased. The ones the lady tried to sew through saddle leather with her touch and throw don't count in my mind. She exploded 4 of the gears in that one, including the 2 vertical ones.
Also, a lot of the newer greases (like the TriFlow grease) are synthetic. Wouldn't that be less likely to break down the plastic and nylon but still keep it from getting brittle?
I also found it interesting that the replacing gears video from SewingPartsOnline shows the techie guy slather a layer of (synthetic?) bearing grease on the nylon gears he just changed.
I'm on the fence and undecided, so I haven't tried anything, since I'd not likely see the results in my lifetime anyway...