Well it can be useful to use the solar/wind overproduction (in the future) from the summer to create hydrogen.
To use it in the winter. Just as a long-term storage. Yes it will not be very efficient, but if you don't have any other use for all that solar power why not?
30% is still better then 0%.
My first thought was that the times when you have the most solar irradiance tend to line up pretty closely with when you need the most power for air conditioning, for the most part, but that's only true for the U.S. If you include Canada, where heat pumps have to do more work heating in the winter than cooling in the summer, that's not true.
So yeah, that's maybe plausible, if we end up using that much solar. On the flip side, if we could get past the geopolitical problems, having massive solar farms in South America for the North American winter and vice versa would solve that, too, and a lot more efficiently than what you'd get from using hydrogen even with transmission losses over thousands of miles. (You'd have only about 35% loss for a HVDC line from Canada to Chile.)
For that matter, pumped storage hydro is dramatically more efficient (70% to 85%) than hydrogen. So is pressurized air storage (also 70% to 85%). And both are a LOT simpler from a technology perspective than anything involving hydrogen, which means they are far less likely to fail, and if they do fail, they're likely to be less catastrophic (read "Oh, the humanity").
I just don't see hydrogen as a viable means of storing energy except in very specific niche cases where you absolutely have to carry an insane amount of energy with you and you cannot deal with the weight of batteries for some reason (e.g. aviation, maybe). For anything where you have a fixed ground installation, hydrogen is only worth doing if you would otherwise be wasting some thermal energy and cannot use it for some other more efficient purpose (e.g. the nuclear power plant hydrogen production example, where you could use waste heat for both splitting water and compressing the hydrogen). If you're starting out with electricity, it's really a non-starter, because there are just too many better ways to store that energy with far lower losses already, and there's no evidence after decades of research that it is even possible for hydrogen to ever reach a point where this won't be true. It probably would violate the laws of thermodynamics.
The only reason anybody is doing anything with hydrogen is because of buzz and available research dollars, IMO. It's a terrible way to do pretty much anything unless there are no other options for some reason, and after nearly a hundred years of trying to make this technology viable, IMO, we'd be better off writing it off as a failed experiment and moving on.