Exercise is great to do every day. The only thing you want to watch out for is overusing the same muscles. Some activities which use the same muscles, like crunches and push ups can be done every day, because the breakdown in the muscle is small enough to heal by the next day. Serious heavy weight training should only be done 2 to 3 days a week because there is enough breakdown in muscle fibers that the muscles need at least 48 hours to heal and actually become stronger.
I wonder about things like running because it's the same muscles in the same plane all the time. Perhaps mixing in biking, or swimming, or an a elliptical or something would be good to switch it up a bit, but that's just my own opinion. I do not know how "real runners" train. I think to them, a 3 or 4 mile run (the equivalent to your 4o min dvd) is like a walk in the park to them, and something they would probably do every day. They would most likely not do the long training runs every day. Again, speaking from stuff I've read and thoughts I've accumulated, but not personal experience.
Lastly, there's how in shape you are and the intensity of the workout. No one would claim we shouldn't go for a walk every day. That's low intensity. No one would probably say we shouldn't dance around our house every day while cleaning or doing laundry, but that could easily be medium intensity. Your JM DVD is most likely high intensity. I'm not familiar with that exact one. If she works certain muscle groups really hard, then perhaps daily isn't the way to go. Perhaps alternate with a few other DVD's as well. But as far as intense aerobic exercise, daily is great!
I'm reading a book right now called Younger Next Year (there's also a Younger Next Year for Women) and the authors promote working up to an hour of aerobics exercise (where you really sweat) every day for the rest of your life to help keep us ahead of aging curve. It's worth a look at the book since you are interested in exercise to a great degree, but the author is not the friendliest. He is a "just suck it up and do it" kind of guy.