Yes, extremely elastic loads are also a good example of where cloud makes sense. In my experience, they're not that common though. Even if you look at something extreme like a Black Friday sale for an online retailer, they're probably not going to see more than, say, 4x the load of a normal day (numbers estimated, few retailers would share them). Also, find me an online retailer that doesn't see Black Friday coming from a mile away and I'll find you an online retailer that's going out of business soon.
What is more common is people deluding themselves about how elastic their workload is going to be, like the startup that claims their business is going to grow 50000% next week. Congrats if it does, but it probably won't.
If you want to brute force some encryption, then sure, spin up a billion cloud servers for a day or two. Few businesses find this a worthwhile endeavor though... except for the NSA, who already has several entire DCs of their own doing this around the clock (making it not particularly elastic anymore).