Actually, rehydrating in a plastic freezer bag is not at all the same as cooking in a plastic bag (which is not recommended except for special boil-in-bags). For freezer-bag rehydration, you bring the water in your pot to a boil, pour it into a freezer bag and put the bag inside a cozy or similar to keep it warm for the 15-20 minutes it takes the food to rehydrate. By the time you turn off the stove, grab the pot and actually start pouring the water into the dehydrated food, the water has lost at least 10* of temperature from its original boiling point (which depends on your altitude). The hot water going into the freezer bag wouldn't be any hotter than the temperature of blanched vegetables put into your freezer bag before chilling and freezing, which is the normal process for freezing veggies at home for which freezer bags were designed.
By using this method I don't have to wash any dishes (big plus for me, your mileage may vary). The used freezer bag becomes my garbage/trash container for the next 24 hours, so it isn't wasted either. I have a medical condition that requires extra wipes/pads/medication for my nether regions, so the used quart freezer bag is just fine to pack out the waste from that in addition to what other trash I might generate.