Yup like Gadget Boy said without the Immobiliser key, no way the bike can be started.
The OEM alarm is extremely piercing and loud and would generally deter the casual punk trying to make off with it or just sitting on it. It was funny watching a friend of mine trying to get off my bike when the alarm went off, he didn't know it was turned on
Oh yes there is bright flashing red light on the rev counter with the OEM alarm also I thought that was a nice touch. Whether or not someone actually sees it in the daytime is another story. Night time that flashing light can be seen from miles.
But generally if I have to leave the bike overnight in a high pedestrian traffic area, I always cover it.