The bird tastes like a bird, retains moisture, and the spices get in. Personally I have a large enough fridge that I can afford to put a bird in a garbage bag in it, so that isn't a problem.
Ref: http://lancaster.unl.edu/food/ftsep04.htm http://www.fsis.usda.gov/factsheets/big_thaw/
None of the text on the UNL page contradicts my suggestion that the risk of chemical leaching is primarily caused by heating.
Definitely don't use garbage bags if they make you uncomfortable. For years, people have also been uncomfortable about doing low temp cooks in Ziploc bags for the same reason (it turns out that this is not a valid concern, but it was a reasonable one until recently). Also! I am sure there are trash bags you definitely don't want to put food in. Some bags are scented. Some of them are stiff and sort of starchy. I'd also stick with big brands.
"The use of plastic trash bags for food storage or cooking is not recommended by USDA '... because they are not food grade plastic and chemicals from them may leach into the food.'"
From Glad themselves: "We do not recommend using Glad Trash bags for food storage."
To each their own, of course. I'm happy to use zip-top bags for low-temp cooking (they are food-safe after all), but garbage bags will be reserved for use in the garbage can.
(I just verified that the bag I used last time suggests it is not for food storage. Erk.)