As a Shaman, it's almost impossible to play a mid-game Sen'jin facing an empty board. Zoo will have minions, and more often than not a way to buff those minions. (Dark Iron Dwarf, etc.) That results in their 1-/2-drop trading for your 4-drop, a substantial tempo gain for the Zoo player. Worst-case, you might end up trading your Sen'jin for a Divine Shield.
Doomguard is what gets used if Zoo has already emptied their hand. Having it down to 4 health afterwards is usually irrelevant; 4 damage isn't always easy to deal as a Shaman, and you'll generally Hex it anyway. It is good if you have an Argent Commander, though. In the end, the trade is only good for the Shaman if Zoo discarded 2 cards for it; even 1 discard is still a decent trade for Zoo, and no discards is a substantial gain.
It's not a bad card for the matchup, but it's not a key card either.