Hazard Emitters applies a heal over time (I think it starts at 17secs at lvl I and can go up to around 30secs) for your hull, adds decent resistances to damage and cleans the ship from any plasma fires.
It can also be used on allies, but only has 5km range.
It is a very decent skill and heals more hull than engineering team, but only over time (so apply it early). The damage-resistances it provides helped me to survive often.
Hazard Emitters 1 repairs slightly more than Engineering Team 1, but does so over time rather than instantly.
It removes various radiation / fire damage effects from your ship (i.e. eject warp plasma).
But the main thing that makes this skill really useful is that it adds a very very respectable hull damage resistance to your ship for 15-20 seconds or more (depending on your skills and emitter gear). 30% damage resistance on your hull is very useful when you've just had your shields torn down.
Also... borg use nasty weapons that cripple your speed and cause radiation damage... Hazard Emitters will remove this.