Comparing with self-hosting or renting a server is just dishonest. At that point you may as well compare with spinning up an actual EC2 instance. Your alternatives miss the main original value proposition of EC2 altogether, let alone spot instances.
Which is?
I don't really buy the idea that companies can't afford to run their own computers due to hardware and infrastructure costs, and that clouds don't require hiring or specialised technical experts.
There’s much less value if you never need to scale of course, which is true of 90% of businesses. But doing a dumb experiment on a 10 cent spot instance for an hour where it would’ve taken a week to arrange space on the company VMWare box? That is awesome.
It's only worth it if the pay-per-second box has an equivalent or better cost-performance ratio than the "legacy" alternative, otherwise it's still cheaper to get one "legacy" VPS/dedicated server, use it for 10 seconds and let the rest go to "waste".