This isn't a decision grounded in economics, obviously. This is about minimizing side-effects from energy production. The sun blasts roughly 10,000 times our electricity needs at us each year. (And that's just what directly impacts the surface.) I'd say we have some wiggle room in which it makes sense to focus on being cleaner rather than more efficient.
We should worry about efficiency if we have serious scarcity issues, but I don't think we really have a scarcity problem at the moment. (We could definitely get there surprisingly quickly, but I think the cleanliness risk is serious and immediate, and we shouldn't focus on hypotheticals.)
This 10,000 times figure is meaningless because it is theoretical; in reality that energy comes in diffuse rays which make it very difficult to capture anywhere near the amount of electricity that is actually needed. Hence the need to turn on coal power plants, which are harmful to the environment (very unwise to have turned off those nuclear plants IMO). Therefore, we SHOULD worry about efficiency, to avoid such things as much as possible.
We haven't turned off nuclear plants (yet). Those which are currently off have either reached EOL (1 or 2 IIRC) or are down for scheduled maintenance or because of unexpected failure of critical components.(pretty much all the rest)
But yes, we actually had some days where we were running without nuclear power. And I don't think it's unwise to turn them off. Few people are actually aware but in 1977 we had a catastrophic failure with a similar characteristic as the Fukushima accident: Block A of the Grundremmingen [1] nuclear plant suffered a power failure followed by a catastrophic loss of main coolant. The block had to be given up since it was not economic to repair and refit it. So while it may be true that a safe reactor can be built, I'm absolutely certain that humans can never safely operate it. We just make too many errors.
[1] Close to Ulm and Augsburg, smack in the middle of Stuttgart and Munich. No more Octoberfest if that blows up.
We should worry about efficiency if we have serious scarcity issues, but I don't think we really have a scarcity problem at the moment. (We could definitely get there surprisingly quickly, but I think the cleanliness risk is serious and immediate, and we shouldn't focus on hypotheticals.)