Unpredictable response times at best, never ending jobs at worst?
If Google starts shutting down your VMs and there's no "bid" process in place like AWS, one has to assume that it does so because there's another customer paying more that wants the capacity or it is Google itself that needs the resources. That means that you won't know when they will become available again, if ever.
So what Google is selling to their customers is the idea of setting up some higher price, always available VMs as a kind of minimum working set and then having the option to pay more for their jobs to run faster, only they can't tell them how much or how fast they will go, the only thing being for sure is that the faster they go, the more you pay.
Not sure how many customers are willing to go for that schema. I can imagine only a few uses (one-off data migrations or conversions, test runs and what else?) where this makes sense, but perhaps I'm not experienced enough.