Both of them are in a band of their own called Southside Johnny and The Ashbury Jukes, so I don't understand what doesn't sound like a band thing is all about.

I do get that there are a lot of Purists out here who don't want what made the band be left out or messed with, but in all honesty, It's pretty obvious that the best reason they're there is to provide backing vocals, and since Richie and Dave are helping Jon out on the high parts it's only fitting.
It's also really nice to hear Bad Name with 2 guitars in it, I like the rythm guitar providing power chords while Richie plays the Intro/Riff.
And they're both from one of Jon's favorite bands so I guess he's getting a kick out of it aswell.