Its better than bolf because its protects your engine minions (auctioneer, knife juggler, pint sized, etc) This card allows for essentially a dynamic +8 health to a minion with a controllable 3 extra damage.
Is it good on its own, no. But can it provide an interesting, positions-based, strategy for gameplay, yes.
Don't forget this card works to protect your minions on the attack too, so you don't even need taunt to benefit. For example, place this down beside a Giant Wasp and the Giant Wasp will take no damage when it attacks.
I think it'll be interesting to see how this works when adjacent to a Divine Shield minion. The damage is reduced to 0 but the Snapjaw Shellfighter takes the damage. Does the shield pop and the shellfighter take 0? Does the shellfighter protect the bubble and take full damage? I'm curious.