Most desalination plants around the world are operating at 100% capacity all of the time, because that's what needed to avoid depletion of the reservoirs.

To be able to "build up reserves" means that you have to build much higher capacity, but only use as much of it as clean energy allows - in today's world, this is highly uneconomical.

If anything, the reserves to be built are energy reserves that allow water to be desalinated continuously -- and that's something which makes sense in general, possibly grid scale as storage technology improves, regardless of the specifics of water desalination.