The term diminishing returns is highly confusing here, since you don't really get less for your points.
Remember, what is important is effective avoidance increase.
When you have 40% avoidance, 2% more avoidance will decrease the average damage taken from avoidable sources by 3.33%.
When you have 50% avoidance, 2% more avoidance will decrease the average damage taken from avoidable sources by 4%.
When you have 60% avoidance, 2% more avoidance will decrease the average damage taken from avoidable sources by 5%.
So while it may take more itemization points to get that 2% avoidance, the results get larger as well.
When I pop my avoidance clickies, I get an extra 10.9% avoidance. This is unbuffed, so buffed it should be less, so let's use 10%.
Raidbuffed, my avoidance should be over 60%, when including debuffs on the boss.
So that means the total amount of melee hits taken goes down by more then 25%, which is a really nice deal. I use them on Gormok, for example. We use a 4stack strategy, and I'm the second tank. So by the time my stacks are growing, the first tank is taking heavy damage, and I want to help the healers as much as I can, so I pop my trinkets a little while after taking my second debuff, to decrease the amount of melee damage taken.
Does it make or break the fight? Hell no.
Does it help? I'm quite sure it does.