I like this article form Fine Home Building- it explains the 'tight house'.
Like I have said before- it's like a swimming pool- you don't want cracks and leaks all over the place- you will just keep having to fill the pool with more water! You want to be in full control of the amount of water in your pool and its temperature. Same goes for your house. You want to be in control of when and where air (and moisture) comes in and goes out and its temperature. If you have a leaky house, you are not in control. Leaks cause problems whether they are air or water leaks.