Spirit Snare: Good gravy, this is an amazing gift. If it had been available back when I was playing Wishbone, it would have been right at the top of my "Must Buy This" list.
By beating a spirit in a mental challenge, you debuff them to the tune of 2 Rage, 2 Gnosis, and 2 Willpower for the rest of the scene. That doesn't necessarily sound like so much until you consider that those traits are on a scale of 1-10, and are used for everything the spirit does. That means that, at the very least, this represents a 20% across-the-board reduction in power; for most spirits, it's going to be more than that.
Remember that in MES, most spirits multiply one of these values by 2 to determine how many traits they have in a given challenge, and very powerful spirits multiply it by 3. Thus, just for starters, this is going to equate to a 4 or 6-trait penalty on all attacks of any kind. It gets better than that, though. Since spirits also use their Rage to determine how much damage they do in combat, this is also a significant debuff to damage; even gifts like Blast are likely to have their damage drop by a point as the result of this gift, in addition to being more difficult to activate by virtue of the debuff to Gnosis.
All in all, this is a MASSIVE penalty for a single gift to impose; I can't think of another gift that's comparable in terms of how much bad stuff it heaps on all at once. This would be an excellent gift as an Intermediate; but they made it a Basic. When fighting spirits, this should be the opening salvo for any Theurge.