Makes more sense to have one app manage everything when possible.
But for me personally my Gmail is my 'If you have this email address you're in the circle' lol... But seriously my other email account\accounts (i.e. provided by isp, my kinda care address and I'll look at it once a week, my i only created this email address so I can sign up for crap things and you can spam me all you want and I don't care address)
So I am one of those that use the two email apps, one for Gmail and the other native email app provided by my isp. My isp was my primary, then I got the evo and I wanted to go all Google, so I've notified my 'circle' giving them my gmail address. All that to say some people might have their reasons