Correct, but how mixed are they? While I try and purge kegs before filling, I don't always. Even if I do purge a keg, I'd assume there's some air in the keg based on the law of partial pressures.
I'm sure the effects of all this are all down in the "noise" though. Add to that most beers I brew are English styles and aren't as O2 critical as something like an American IPA.
I was just looking for a simpler way to vent the keg from the top of the keg lid, rather than a few inches down. It just makes more sense to vent from a different spot than when you fill. Plus, it allows me to hook up the gas in line once, rather than hook up gas/hook up bleeder tap/repeat.