I saw profHulk using it an macro boghoppers BG, was pretty effective. Don't know about other players though
I've been using her, she's pretty good overall. Decent damage, strong support abilities. Not really seeing what she'd add to FW either, honestly. If you don't like her abilities in FS it's not like they'll suddenly become good in FW. The only thing is that FW can make use of Class: Witch slightly, but that's not really saying much.
To mention (some) of the benefits. One could look at death benefiet withthe charges, ap generation (potion), equip with bracelet of mirrors or the equip that gived spell charged, it heals (which fw (now) has access) to anyway. Cleanse. Probably missing a few things.
But the point is that FS can take advantage of most of that anyway. I will grant that I forgot about Bracelet of Mirrors though, so that's some extra charges. Then again that's an argument for putting her in UD as much as FW (and you could opt for Murder then too if you wanted instead), or just running FS/FW (or whatever) splits. FW has more (useful) Short Lived minions and all that, but FS also has access to Resurrection and the like, and doesn't actually lack ways to sacrifice their own minions if they really want to. I'm not saying Darkmarsh Witch would be bad in FW (she'd be good), I'm just saying that, compared to FS, there isn't that much of a difference, so if Tuggles doesn't like her currently in FF FS then there's not really much gained by putting her in FF FW. *shrug* Just how I'm looking at it.
Using her on a full FW BG would be way more profitable due to the full FW faction bonus. I play a FW/FS split and I can get what... 10~12 charges on a 30 min match?(considering that I pull of successful muckers giving 4 chargers, it would be more if Timothy wasn't nerfed) Even with that, she doesn't even compare to what Keeper of Memories used to do. I feel she can't reach full potential like that. Giving split to her on this case would give more options of BG building, your opinion even helps because it shows she wouldn't be gamebreaking, as you implied, it wouldn't change much. Also, let us not forget this would be an apology for the Tortun blunder.
I basically want her to be FW/FS so I can split her into a FF FW witch-ish deck. She has a TON of utility which FW kind of lacks, and with how liberally I use repurpose/reanimate she would be a great addition.