The hard-core competition folks are looking for any, and all, "legal" advantages to make that one or two bites the judges eat as spectacular as possible; phosphates are one of those tools. There are several commercially-available competition injections that use them (like FAB-B, which is full of MSG), and are typically pretty pricey. FAB-B is around $20 for enough to do 3-5 briskets!
Personally, I don't think these "extreme" measures are necessary in our smokers. Competition, cooking on a hot stick burner? Maybe so. Tight, moisture-retaining SI unit? Not so much. I think we have better success with our traditional brining/injecting/rub methods, combined with the stellar cooking environment of the SI, to need extreme measures like phosphates. I like to keep my ingredients natural.
