Ziploc claims to be BPA and dioxin free. Probably true, but what else is in there. Plastic breaks down with heat. Very well is not an issue, especially just for a week or so, but I find it easier to just pack a few of the mylar bags. There's plenty of crap I admittedly put into my body, but I guess we all have a line to draw.
Another con for the ziploc cooking is the propensity for the rigid food to poke holes in them while bouncing and rubbing around in a pack. I have a hunting partner that uses that method and 1-2 times a trip he pours the water in to find it leaking onto his sleeping bag or clothes a minute later.