When you can't get synergy with Target Dummy it's half a Mirror Image, that's laughably bad but still eats up a slot. Doesn't even work with Hobogoblin. It's like running Wisp in Rogue because of the combo mechanic.
I think its secret strength is being a taunt. Even lategame a taunt can help you race and it costs no mana so you can do a lot of other things the same turn, like deal a bunch of damage. So a bit more useful than wisp late game to say the least.
Also in a rogue deck like you mentioned, but it does bring other things to the table that wisp doesn't.
But I lean towards your opinion of it simply not being enough value to justify since getting that extra synergy is pretty situational and without it, it probably sucks. I could see experimenting wiith it though. There are times where I could really get high value out of the new 4 mana combo spells on turns 4-5 that are immensely difficult to combo with that much mana.
What's the current consensus on Mogor the Ogre and Hemit Nessengwary? I feel like both of those cards have interesting possibilities, but at the same time they both seem a little too situational to commonly see use in decks. I opened 40 packs last night and those were the two legs I got. A little disappointing to be honest, I was at least hoping for a top-tier seeming legendary like Vol'Gin or Mal'Ganis, but hey, at least I got something.
I think mogor is more of a defensive card. You play your turn out and attack with everything, then play mogor and your opponent is going to have a difficult time removing your board with minion attacks unless he can deal with mogor first.
Nessingwary is definitely a meta specific card. When druids and hunters are running out of control with beast decks, I could see him being used. DOTC and druid of the fang are both beasts after they transform.
If you're thinking about dusting them, I'd personally keep them. I would dust some midtier classic legendary cards before dusting any of the new ones.