Processed food is not healthy. Period. It might not kill you right away, but it certainly causes problems. Those who can afford alternatives should do so as often as possible.
Eating healthy isn't a "Yes" or "No" proposition -- there are an infinite degrees in eating healthy.
We buy processed chicken sandwiches (or hamburgers) for $1 at a drive-thru on the way home from Mass every Sunday. We live too far away to make it home, and it's hard to bring "real food" with us without a microwave around. Sure, we could bring frozen food and use the church's microwave, but how would that be any different (health/cost) than buying fast food?
Anyhow, we try to eat healthy as much as possible, while forgiving ourselves a few exceptions. It's not like if you can't "100% healthy" you should just "give up" and take up smoking, alcoholism, give up exercise, and eat 100% processed food, and eat lots of foods with MSG and Aspartame. I think that would be borderline suicidal. Can a Catholic "give up" in good conscience and not take care of his health AT ALL? I don't think so, not without sin.
Matthew