Yeah I expect acting as SSD bays could become popular in the future if SSD prices drop low enough. Although they might be M.2 bays by then.
I have a bunch of old 60 GB 2.5" SSDs around but they’re so small it’s not worth bothering to set up an array of them. Plus they’re more useful individually for stuff like upgrading an old laptop, portable USB storage or installing Windows the one time in three years I need it.
In the meantime I’ve liberated the 2x HDD cage from a Define C Mini’s shroud and mounted it on the floor in a fan slot.