Plenty of use cases...
- batch processing of all sorts
- auto-scaling (just launch two at a time when you only need one, so that it doesn't hurt if one goes down, and still pay less), obviously much more efficient and feasible at larger scale
- automated testing
- etc etc
The problem with Google's offer in comparison to AWS's Spot Instances is that it's too expensive and limited to 24 hours max.
In AWS you regularly get spot instances for below 20% of the normal price (just tested with a m3.large instance for $0.03 per hour, instead of the normal $0.154 per hour; became available within the normal time frame... two minutes).
That said, prices usually spike dramatically when there's high demand, which means it almost doesn't matter how much you bid. If the spike kicks in, your instance goes down.(Spike prices are often higher than the regular instance price, so unless you bid insanely high amounts, your instance is going to go away.) That's fine though, because you get serious compute power for very low prices, and you know it's not going to last. Design your application to cope with it and you can actually save quite a bit.