As far as I know, they do use it for baseload, but there's a very big difference between baseload and peak. In the summer in New Hampshire, for example, it's very common for people to run their air conditioners for a few hours during the hottest part of the day, but turn them off the rest of the time because it's not miserable. If you're fluctuating between peak and no heating/cooling need over the course of every 24 hours, that peak demand is a real problem, and demands a lot from the other energy plants.
If you could ramp the baseline production up a little bit and meet more of the peak demand from nuclear, it'd be cleaner than relying on the other plants, but you end up with too much energy produced, especially at night when it's cooler. Conveniently, cool temps are a benefit to bitcoin mining rigs, so it works out to spin up some ASICs overnight and make some money from the excess energy.
Nobody's suggesting the utility hold the bitcoin. The expectation would be that it would be sold whenever it's generated, so there isn't really any financial risk. But even if there weren't money to be made from it, having something that can easily be ramped up or down to compensate for energy demand is still useful.
