Well, researchers looking for a cure to AIDS use this bioluminescent gene to "tag" cats that have taken a special protein to prevent them from being infected with FIV, a close relative to AIDS (aka the "cat form" of AIDS). Without these tags, it is extremely difficult for scientist to know when the cats have taken in the protein.
PROS AND CONS OF GMO'S
GMO's can be used in our crops and livestock to prevent them from contracting certain diseases, which, in turn, would provide us with more food. If we get more food per dollar, then the prices of food would drop drastically. If these prices drop, then more people who would normally not be able to afford very much food, or are on a budget, will be able to afford more food rather than starving themselves. Also, if these are incorporated into our foods, then we could find ways to create animals and crops that would provide more nutrients for our consumers, which could help with the world's overall health.
Since GMO's are fairly new, we don't have the luxury of knowing the long term effects of them over time. Another reason that they could be a bad idea, is that we are creating all of these perfect organisms who are meant to not die easily, so if they are accidentally let into the wild, then they could become invasive and destroy whole environments. Plus, people are wary of mixing organism because of all of the allergies people have, and they don't want new allergens being produced or old ones not being labeled correctly.