Already fourteen posts on a thread dealing with a missing washer. I really like this forum 
I think, to be on the safe side, it's good to assume that Honda did not put any part into your bike that is just unnecessary. In case of this particular washer, I guess that it prevents the spring from ripping small rubber pieces out of the oil filter while spinning on the bolt. These pieces would be pushed right into the main oil gallery possibly clogging a passage to parts of your motor that don't survive without good fresh oil.
This seems to be a very rare event, but we've seen earthquakes, tsunamis and three exploding nuclear power stations in a row...
I agree.
It can be argued that we don't even need an oil filter at all let alone one that works perfectly, if we are careful about oil changes and keep clean the things we put in the oil system, and if no parts inside start shaving metal, and so on.
In this particular case, the end of a spring with irregular contour and sharp edges pushes against a soft rubber device that seals between filter and mount bolt. There is the possibility sharp spring components can "shave" some rubber bits, and that the seal will distort and fail to seal, allowing oil to bypass the filter. If you think a filter is important, than it stands to reason that you think having it work on ALL the oil, rather than just a portion of it is also important.
Lots of items are on the bike as a precaution or for the purpose of prevention. To say they are always or never used/required is ill informed.
I have a car with air bags and I've never used them. I've never had the single throttle return spring break on an SOHC4, so as to require the pull closed cable. It doesn't mean I will never need these items, or have good benefit from these items included on the machine.
Engineers aren't paid to put parts on or in a machine that have no purpose. Quite the opposite. They take all the parts off the prototype that aren't needed in production, (or required by marketing concerns). For Honda, if it is inside the engine, it has a purpose, whether you are able to divine a reason or not. Could be you are just not informed enough to understand truly why. Engineers pay their own salary by taking costs out of a production run. Saving a dollar on each of a production run of 1 million units, is a million dollars toward profitability.
It does give me some amusement when "customizers" remove and change things because they don't understand them or even care to understand the purpose and function. But, it is sad when someone unsuspecting buys such an altered example. IMO
Cheers,
