As a common user of a MUT in the tricities (very near Yakima), I have learned to NEVER say "on your left". Even the cyclists using the multi use path don't know what that means. I just say "good morning/afternoon/evening". That works fairly well. I also slow down and watch.
So, I fully agree, something should have been said, but "on your left" is dangerous in that crowd, they just don't know what it means.
I was riding home from work with 3 guys that work on my floor just last week, and I told what I thought was a funny story about the final time I used "On your left" and these 3 guys, who have been commuting on this trail for the last 3 years - not one of them knew what "on your left" meant! I told them it was common cycling courtesy and all 3 of them - pure commuters - thought I was nuts.