I think this compares somewhat poorly to Leather Coat coat and I'd rather take that if I can. Waiting for 3+ spells over 3+ turns to just breakeven makes this a hard sell. Sometimes you draw it late and you arent playing any more spells. Even early on, it feels slow to play this, gather more resources and play your main spells.
But not every can take leather coat. Robes of endless night does however compare favorably to the neutral Trench Coat and decently to Heavy Furs. Only Sefina has enough agility to even think about evading but she is even more heavy on spells.
Heavy furs might be useful in Diana Stanley and maybe Jim Culver but for the most part, the economy is better than an extra reroll because mystics usually do want the health soak for themselves. The reroll on symbols is occasionally useful to avoid the downsides of your spells but mystics have other avenues to avoid that.
It's generally obvious when you'll take this, a mystic with low (5) health and lots of spells (maybe around 10 spells). With around 10 spells, you should be able to play 1 every 2 turns and make back your cost after 6 turns (about 1/3 of the scenario).