There's three types of workloads that make sense to run on EC2:
a) Extremely spiky/seasonal loads (batch jobs, event/campaign traffic)
b) Loads that can be structured as to run entirely from spot-instances (worker-pools)
c) Loads so small that the markup versus rented/dedicated hardware just doesn't matter
Maybe I'd just add one more case here: some users are OK with locking themselves up to AWS by treating it as a platform from the day one and building on top of AWS database/queue/etc services. For those people using EC2 just to run the app code and replacing instances when they misbehave may be a good idea.
And to your point somewhere else in here, it is a hell of a thing to try and move away from that platform. Yeah, it's super easy to beat EC2 on cost of compute resources, but really if you're running everything yourself on top of compute at AWS then you're doing it wrong.