Beyond that, though, isn't empty space encryption far more important as respects SSDs than it is as respects HDDs because of the fact that SSDs retain data even after they've been "erased"?
Who said magnetic HDs don't retain any data when they have been erased? It all depends on how badly someone wants the data. Even the 7x overwrite is NOT foolproof. There are labs that can go deeper than that. The only foolproof erasure is a grinder that reduces the platter to dust.
While a 7 pass overwrite of a HDD is, as you say, nowhere near a panacea, a 35 pass overwrite, which is readily available in appropriate versions of Disk Utility, is pretty close...closer, to the best of my knowledge, than directory erasure which leaves our data intact, our only option with SSDs.
As you recently said, "[As Virtual1 so clearly pointed out,] it only takes a tiny wedge to enable crypto analysts to get the data if they want it badly enough."
