Sharing a joyful life moment with the world vs. showing your, as Oprah would say, Vajayjay, seems to be a completely different thing to me. That's just my opinion though.
If I were them I wouldn't choose to have my family on tv. However, I think their motivations are different than Jon and Kate and many reality show families at least. The Duggars say it's not mainly about the money, but about sharing their beliefs and life style. This is a way to do it with a wide audience.
They don't seem like many other families we see who are after attention, money, fame and noteriety.
There are a lot of religious people who want to share the joys they have found in their religion/lifestyle with others. The Duggars say they want to share that and their show is how they do it. They aren't trying to force us to live the way they do, just showing us the joy that it brings them. So far I tend to believe that is their motivation, but I guess we'll see what happens.
I don't have a problem with it so far and actually think they are a pretty good example of a loving family.
I have heard people say they've been inspired by the show to improve their families, live more frugally/greenly, yell at their kids less (has Michelle ever raised her voice in anger?), spend more time together etc... They have made me want to try some things to conserve cash, like making my own laundry soap as they do, it's cheaper and greener.
As I said I would certainly never put my family on tv. I hope it works out well for them. I hope they are thoughtful and careful about it. Overall reality tv doesn't seem to do much good for families.