The funny thing is, if you *try* to use it effectively, by berserking some Thugs/SCs, it tends to fail. The caster will either select another far away staple of units, or cast something entirely different.
To minimize range and thus the ability of the AI to select unwanted units, you sometimes even have to take your lesser nature mages into battle, or not research too high. Because with enough gems, they'll still cast the more powerful version.