Earlier quoted context omitted.
Except if I have batteries, why should I charge them with expensive nuclear energy when I can charge them with cheap renewables? The nuclear plant will be forced to compete with those renewables for this market, which will limit what it can earn with the otherwise curtailed output. This is not as bad as losing it entirely, or even paying for someone to take it, but it's still going to be a net negative for the plant'…
Do you have sufficient battery capacity to power your grid through a 90th percentile worst case scenario solar outage? 95th? 99th? How long can it go without running coal and gas? Nuclear works when you want it to. Solar and wind work when they want to. That's a very big difference when you're producing the electricity people rely on to live their daily lives.
Hydrogen can be much cheaper for (say) the last 10%, because (1) hydrogen has very low capital cost per unit of storage capacity, and (2) the efficiency hit of going through hydrogen vs. batteries is less important when it's just 10% of the total.
Think of batteries and hydrogen as analogous to cache memory and main memory in a computer. They have different performance and economic characteristics and compensate for each others weak points.
To see this in operation, go to https://model.energy/ and try turning hydrogen off and on in the settings. If you simulate for Germany, for example, turning off hydrogen can double the cost of achieving a certain level of constant grid power. Hydrogen can be particularly valuable for places with large seasonal variation or lots of wind (which has a long timescale component in how it varies.)
I will add that China is already selling electrolysers for < $300/kW, less than half that simulation's 2030 cost assumption.