It seems that the nozzles feeding onto the aerospike are sufficiently small to operate safely at sea level air pressure. That kind of nozzle can operate safely at lower air pressure as well, but is inefficient. The aerospike gives you back your efficiency at lower atmospheric pressures, so you end up with something that's both safe and efficient at all altitudes, relative to a traditional bell nozzle.

At least that's my understanding - I just like to learn about rockets.