Seems wasteful. They could at least turn them into driders or feed them to the spiders. Hopefully they torture them to death for the glory of Lloth at least
On the Disc it's the 8th son of an 8th son ("8" being the more magical number, along with the eighth color only wizards can see, octarine). The 8th son of an 8th son of an 8th son is a sourcerer, but they're rare (on purpose).
In fact, the Kafes of the Ottoman Empire were an effort to stop the ritual of a successor king having all his brothers executed. Instead, extra heirs would be locked up in a nice dwelling full of harem women. They went mad.
And now they are a tourist attraction and a footnote of history.