Insidious Dreams is an up-to-26% damage mod, but only for Magic-based nukes (not mentioned by Alla's). The focus effect is thus useless for Shamans and Druids and Necromancers, who don't have a MR nuke line.
Looking at other classes, you have to consider where these fall, and the options. Clerics will infintely prefer the Manaskin plate-class boots, thus eliminating all the priest casters from real contention for these.
Given Magicians' predeliction for FR nukes (way better than the mr line, esp with Ancient:Shock of Sun), and Wizard general preference for Cold, and wide choices regardless, these shoes become all but exclusively for Enchanters. The name makes sense, the stats make sense, all is right with the world. My guess is that the other six classes were added to the item just so that they wouldn't rot once you get enough for all your chanters.

