Rubber plugs missing?

I recently replaced some shims on my '93 Venture and I noted after removing the front valve cover that 4 of the cam cap rubber plugs that regulate oil pressure were missing. Looks like the the plugs have been missing since the last time the previous owner removed the valve covers. I am not sure what issues can be caused by not having these plugs in place. The bike seemed to be running fine before doing the shim replacement. Seems kind of odd for so many to be missing. Would there be any reason for the previous owner to leave them out? Is there any damage that can result from this condition? Hoping someone can shed some light on this situation.

Thanks

Bobber