In my opinion, if someone says we should just have all nuclear, it means they are dealing in absolutes, maybe God in politics and debate, but they ignore that reality is much more complicated, as technology meets economics and politics (and much more).
(Again, AFAIK) nuclear energy is great when it comes to meeting the "baseline" requirements, roughly the level below which energy consumption of a region never drops. It is because nuclear energy cannot be scaled up and down quickly, so it's not adequate for peaks.
To balance the peaks and keep the network stable, you need energy sources that can increase their output in seconds (or less?).
That's also one of the issues with wind and solar, you can't choose when it is available and when it isn't, so coal etc is still needed to make sure that energy production and consumption is practically equal at all times.