Absolutely - a signal mirror should be a backup signaling device, not primary.

You want some form of 24/7 all-weather radio means for calling help, ideally a PLB. You will summon help much more surely and quickly that way. PLBs/EPIRBs contributed to the rescue of 1,596 people in 2009 alone, and in areas with good coverage, the cell phone is probably the first thing to try. As a licensed amateur radio operator, I also carry a 2 meter radio with builtin GPS.

Most recent signal mirror rescues occured because the primary radio system failed - malfunction, batteries, crushed in the fall, or went down with the boat.

Any visual signalling method is far from a sure thing - survivor's stories are rife with tales of ships and planes that blithely passed by flares, smoke, and signal mirror flashes. Signal mirrors also require coherent illumination, direct line of sight to the rescuer, and are easily mistaken for natural phenomena, unlike, say, orange smoke.

Mirrors, however, have an infinite shelf and battery life, are not expendable, and are extremely light and compact. After you've run out of everything else, they are the thing to try.

I focus on signal mirrors (somwhat quixotically) because I think that, while their utility is limited, there is a lot more potential there than is realized.

Where signal mirrors could be useful, all too often, people don't have a mirror, have an inferior mirror, or don't know how to use the mirror effectively.