seems dumb to have electricity needing to be wasted when there is seawater to desalinate
That's a much more complicated problem. On an energy market, you have only one price to look at, and the battery operator can always buy, sell, or hold energy. The article here talks about optimizing this problem at 5-minute to several-hour intervals.
If you drop excess power into desalination, however, now you have two prices to worry about: energy and water. I also doubt we have 5-minute spot markets for water, so the operator must probably commit to some medium-term water delivery regardless of price.
This means that a desalinating firm takes on much more risk. This might still be profitable, but it's a long-term play based on a deep model of expected energy prices (i.e. knowing that energy is "always" almost free at noon in summer) rather than short-term time-shifting.
Why couldn’t it just be a giant heating element and some sort of steam condenser at the top and some way to flush it periodically?
It might burn some laughable 3kWh per kg of water, but who cares? every water utility on the coast could add a few megawatts of tea kettles and get opportunistic little splashes of water in volumes small enough they can probably already handle them and the brine discharge would be so small, disperse, and infrequent it’d be easier to deal with, and it’d basically cost nothing
Even free power would likely not be worth using if it was sporadic, and it’s extremely energy intensive. So that really is saying something.