I used to use cloak with erebus all the time, and I was a top 10 player without stacked teams or exploits...
Yes, you have bat swarm but the problem is that's a one-way ticket. If you _really_ have a good feel for what you can and can't survive, you can bat swarm behind enemy towers to a fleeing opponent, get a bite off, finish off with auto-attack, and still TP out without dying to towers.
You need to have a very good feel for your character to do this, since if you don't have enough mana leftover after bat swarm to bite you're often dead, and if you misjudge the health differentials on either character, the number of auto-attacks (hopefully 0) you'll need before bite's slow wears off, or the tower damage you're dead. Against good players, however, I found that walking the razor's edge was often the only way to gank quick kills that would turn the game.
Also nice for if bat swarm is on CD or you're low on mana.
Finally, it's just more exciting and fun. Health stacking is lame and boring and once it really took root it killed the game for me so I left.