When I first glanced at the ability, I thought that it was a nod to the 3.5 Spellthief, although why such homage would be paid to one of the most universally reviled 3.5 classes I don't really know.
However, after reading, and rereading, the ability, I think it could have situational usage in debuffing a Wizard or Sorcerer, especially if you know what spells to target.
I would use it primarily against a Sorcerer (or other Spontaneous Caster). For instance, Sorcerer casts Disintegrate. I probably would rather her not cast that one again, so I steal the spell. Now she can't cast it, but I can. Double win.
Like I said, I see this as a debuffing tactic to use against Spontaneous Casters... once you see their big gun spells, you steal them so that they can't reuse them. So yeah, its a pretty situational ability. FWIW, this was the big gripe against the Spellthief. The class was actually very good at what it did, but what it did was so hyper-specialized that, unless you were playing one in a very high magic game, you were pretty-much just a weak Rogue with half sneak attack, bab BAB and a terrible list of weapons. Basically, it was a feat tree turned into a 20 level class.
