Things have changed a bit here, and so I'll throw my two cents in for wither. There are always better options.
Yes sure, in theory with Arcane Research, its effectively a 2xp spell, but in practice it's not. To use arcane knowledge you have to have Wither which is just awful, and for most mystics in most situations is a blank draw, or worse a 2 resource 1 actions sink. So until you can upgrade both you lose 2 decks slots. Now let's say you know this and decide to use your first 6xp to get two Wither, then and only then is it somewhat viable. Not to mention using your Arcane Research on Wither instead of Shrivelling or Azure Flame is lunacy.
The cons to Wither:
1) Costs 4-8xp. 4xp means you have to waste the first two scenarios on Wither taking up valuable slots.
2) Takes up an arcane slot. If for some god forsaken reason you have been chosen to be the fighter as mystic, then you want both hands for Shrivelling and Azure Flame, which you can upgrade for 8xp by scenario 3. If you want something without charges then choose Sword Cane, though you lose two will.
3) Spectral Razor exists for the same cost, and action though its 0xp and can hit for 3, instead of maybe 2 likely 1.
4) Requires you to pull a specific token or does 1 damage, and doesn't help your next hits, essentially not doing its job.
Now I know this seems grim, but there are some pros that make it sort of viable?
The pros: Wither
1) Infinite use, your the fighter and need an infinite use back up weapon but also need +2 will to consistently land hits.
2) Want to only use wither (to save deck slots) to assist your fighter by going first getting off wither hits (or misses) but dropping the enemies fight and health value.
3) You have built an entire deck around it, though it is not at all powerful enough for this to work well.
At the end of the day you have to look at your options, instead of upgraded wither what can you do for; 8xp, two spell slots, and two decks slots, that could help you win the scenario. The answer is a lot. Drop wither for more test-less clues, for upgraded allies, Charisma and summoned hound, Twila Katherine Price and Charisma (1xp more and gives infinite Shrivelling, Charisma and Dayana Esperence + Spectral Razor, the list goes on. Wither is never worth the price of admission, nor the cost of building a deck around it, though I would be willing to admit that Jacqueline Fine already builds decks around revealing tokens, so there is natural synergy. For every other mystic there is always a better option.