--I have a large pressure canner that *could* be used for pressure cooking meals but it is very large (too much food), difficult to lift when full, and I wouldn't use it to pressure cook a meal for the most part. I can CAN 7 quarts of food in it--say turkey, venison, beans, corn, tomatoes. Generally, the pressure canners come with a rack to hold the jars off the bottom and so they don't touch each other.
--If I wanted to pressure cook a meal, say soups, meats, stews, I'd buy a smaller meal sized pressure cooker that is too small to can in. The electric ones sound really nice.
--So ideally, I'd use one as a canner (large) and one for meals (smaller).
(Unless you find a multi pressure cooker canner that is the right size for your needs.)
--about your question Adillo about the downside of using a Pressure Cooker/Canner for meals is, that when you clean a 21 quart monster of water, it's not too hard, when you clean it up from beef stew, it's a heavy and messy job to clean up. Mine for instance doesn't fit in the sink.
--about your question of using a PC for a water bath canner, you can use any pot for water bath canning if you have a lid, and the PC lid can be set on top (though not tightened) of the PC while you water bath can. (when pressure canning, fill the water to half the height of the jars, when water bath canning, the water should cover all the jars and lids)
--about your question of how much time does it take--the larger the volume of water or foods, (the temperature you start with matters) the longer it takes to heat up. My PC fits over two burners, takes about a half hour to start steaming, more to throw off steam in the vent, (close the vent) more to come to pressure, (adjust the heat) then the time it takes in the recipe (20-90 minutes), then turn off and slowly start letting off pressure (open the vent). I almost always have to set aside 3 hours for simple preparation and canning to do a batch of anything. I hope that helps ~Bliss