Finally, a body asset, and a quite strong one at that!
First off, damage soak can always be appreciated by a faction that tends to be low on the health side, but for that alone it's fairly expensive. On that merit, it's hard not to compare to Trench Coat, which costs the same, but provides a bonus that most Mystics will find useless. There's also Leather Coat, which provides the same protection for free, and synergizes with recursion (if that's even something to consider in a deck).
But the true strength of this card is in its economy. After three rounds of playing spells--which is just what Mystics do--you break even, and from then on you're in the black. As long as you play this early and you're on a good curve, that's better than a 0 cost card.
How does this compare to the mainstay economy card Uncage the Soul? Well, the discount here is plainly inferior (on a per-card basis at least), but Uncage has a major downside: it's bad for playing events. Most Spell events are either fast (meaning they either can't be uncaged because they have a prereq, or you're trading an action), or are used to Fight/Evade which is bad to "play a card" over. The Robes, on the other hand, are a nice clean . Even the lesser discount can be good, as there are a handful of spells that are too cheap to waste an Uncage on. And, so long as they don't get discarded, the Robes can be used way more than twice!
I think Uncage is still better for most assets (and it's got that nice ), but running the Robes with it sounds like a great way to almost guarantee a strong economy for a faction that typically likes expensive decks.