50, 40, 30 is code for "here comes 20 and 10." Seriously, they provide a trend with the rate they are called. "504030", when compared to the usual "50...40...30...", lets you know, when your eyes are outside, that you have a high rate of descent. Yes of course you shouldn't need it, but it's another little piece of the situational awareness puzzle.
I agree it can be a distraction, particularly when the callouts in different aircraft don't quite match each other and you have to remember, "is this the one that calls 10 too high?" It's easier to land without them than it is to ignore them.