It certainly increases the safety factor, but should never be relied upon as the sole redundant backup system. There are situations where it will not do you any good such as hovering at low altitude. If the aircraft does not have a reasonable glide slope, most people would feel a lot more comfortable with one.
The Lillium aircraft has no glide slope at all. It has wings, but there are no control surfaces to maintain a glide in case of power loss.
More cost.
An airframe parachute is included in the Lilium design. At least it was last time I checked.