The possible reason for Apple to remove the headphone jack? Mechanical simplicity. I have not seen it discussed yet, but it seems all the analysts and pundits are missing this point.
The dock connector can output audio so the headphone jack is technically redundant. From a recent iFixit iPhone tear down, the module that contains the Lightning connector, mics, speaker and headphone connection is a single unit that has to be replaced if any of the components fail. How do you improve reliability?..Remove a point of failure.
I'm sure Apple has statics on the failure rate of the headphone jack on devices. The cost to repair this failure can be significant amount compared to the cost of a 3.5 millimeter headphone jack. Removal of this part can have an impact the amount of money and time Apple spends on repairs via replacement of the part or worse, the whole phone.
Other benefits for removal of the headphone jack will be reducing overall manufacturing costs and making the process more efficient. Once again simplicity is the key. By not having an additional step to join a technical redundant piece of hardware to the board, manufacturing speed and quality can be improved. Over the manufacturing lifetime of tens of millions of iPhones, parts and labor savings will be realized.
Apple is an unique technology company because they never felt they had to maintain a status quo. Through their history they have made choices that have been initially rejected. The GUI, the mouse and the iPhone's software keyboard that the world would never figure out were advancements that eventually benefited all. The concept of innovation, especially in the tech world sometimes has the expectation of a magic trick. Apple's magic, has been making things that we really didn't need, disappear at the right time.
I just read Marco Armet's post from July 1. He states that minor savings in parts and complexity as a benefit. But multiply that minor savings by the 232 million phones Apple sold last year and I feel this change will have a bigger overall impact.