It's not inconceivable that a plasterboard ceiling will collapse before the fabric of a building degrades to a point where the fire service won't consider entering the premises, particularly if they are inside fighting the fire with water.
If that happens, the cables you've decided aren't subject to premature collapse because they are supported by the plasterboard could come raining down presenting a risk to fire fighters trying to enter or leave.
A couple of quid, a few clips, lengths of all band or whatever seems a small price to pay for potentially making the installation safer for those who choose to put themselves in harms way so that those unlucky enough to be caught up in a fire have a chance of surviving the ordeal.