It appears you are mixing two ideas, /targetrandom and /castrandom. As to /targetrandom, I don't know if this even exist but if it did it would be a really bad idea. /castrandom on the other hand can be very handy particularly to use trinkets and/or spells with longer cooldowns like Elemental Mastery. Some people will disagree with using it for things like EM because it is not at your disposal when you need it, however, I use it way more this way than I would if I had to think about it all the time. But I'm not sure if it would be a good idea for something with a short cooldown such as Chain Lightning especially when there are multiple mobs and/or adds where a CL could rip aggro before your tank has a chance to gain it.