Do the washers do the same job of reinforcing the holes that the eyelets / grommets do? (I know it's probably not much, but anything that distributes the forces better to minimize cracking at the hole under stress will minimize spoke tension loss and failures).
The eyeletted rims I have used in my cargo applications have had far less problems with spokes (just two broken, IIRC, both on a single wheel) than non-eyeletted, but I have never had the opportunity to setup a test on otherwise identical wheels with and without, under the same conditions, for wheels that I built and know the conditions of, just to see what happens.
So I don't know that the eyelets have anything to do with the results, but:
The one noneyeletted wheel I used on the trike's rear end in recent times is the only one that suddenly and completely failed; I don't know what previous stresses it had seen, and I don't know how it was built, etc., and I couldn't see any defects around the nipple holes, so there could be several unrelated reasons for the failure.
Anyway...just curious if it could make any difference to the structural integrity of the rim around the holes, especially in high stress applications like mine.