Well i'm sure you would all like a 4 mana 4/4 or something like that, but Rogue minions usually have higher attack minions, that's kind of a tradition now. Or can i use the usual wording, "class identity".
Yeah I don't think the 3 health is a huge deal. Card is clearly designed to be played the turn after Shadowblade I'm just not sure using your Turn 4 on such an easy to kill minion for the sake of one durability is actually worth it. It's not going to survive.
Turn 1, opponent plays a minion, you backstab it Turn 2, opponent plays a minion, you use backstab/coin/HP/eviscerate whatever Turn 3, opponent plays a minion, you pull out the shadowblade, remove it Turn 4, opponent plays a minion, you drop this fella, attack the enemy minion but 3 damage is to low to destroy it Turn 5, opponent plays a minion, destroys your runeforge haunter, you have no minions and no weapon in hand, you concede
5/3 is really weak, even with a followed up Crazed Alchemist it's still weak. But it might be worth the 4 mana for an extra swing of Shadowblade to finish a big minion, in that case the 4 mana are a good invested pseudo-heal with a 5/3 body. Very situational, yes, but I think Shadowblade will see play so this card might be put in decks alongside with it.