I use some of those vertical chicken roasters when I'm cooking BCCs. They're built to put a beer can inside the part that goes inside the chicken and they work very well. But I've forgotten the can inside a few times and like p-nut, I found that the chicken stayed just as good.
So maybe it's just the fact that the chicken is on some sort of stand that makes the difference. I know it isn't brining that makes the chicken juicy either, as I usually cook mine unbrined and I've found it no different from brined chicken either. (I only brine older chickens as they actually do tend to dry out more than the younger hens.)
But hey - barbecue and grilling is all about the freedom to cook however you feel like. If beer in the can works for you, then go right ahead and use it. If not, then that's OK too.
There really are no right or wrong answers if something works for you - just different ways to cook. I've got a brother-in-law who still boils-and-burns his ribs and he swears by them even though he likes my ribs even better than his. Go figure.