The simulations are really cool. I'm trying to think of a real-world situation where you'd have differently-sized backends though. I guess it might help if your server runs hourly cron jobs that take away system resources from requests.
I don't think many people are intentionally deploying their apps with replicas of different sizes, you're right.
But this is it: noisy neighbours, inherent physical differences in even identical hardware, using different node sizes in your clusters. I think incidental differences in servers are very common, even within the same AWS instance category.
I thought of another variant: with AWS autoscaling groups using spot instances you can list a number of different instance sizes and say "give me whatever is cheapest" and you'll often get a mix.
