Agreed. You would only sell if there is a major advantage to doing so.
One advantage is to recoup a lot of sunk costs to release much needed funds for other, more profitable, opportunities that are available to you. Given how profitable AP is likely to become once it is successful I think I would be challenged to find another more profitable use for my funds. Maybe a desktop cold fusion power plant that runs for 10 years on a single charge of heavy water? I don't think this is a compelling argument simply because of AP's potentially highly profitable revenue stream extending into the future.
Another advantage is to do a shares swap with another, even more profitable company. Fred allows Apple to buy the company and exchanges his holding in AKP for shares in say Apple or Sony, knowing that he will make more out of owning Apple or Sony with AP speakers in them, than he will out of owning AKP. But as you point out AP could well make substantially more out of its products if they go into all of the myriad of sound generating devices rather than just into Apple or Sony products. I don't think this is a compelling argument either. It underestimates the potential of AP speakers to significantly change the entire audio market forever.
Both of these arguments apply to Beats. As @jonfoulkes points out Beats is nothing really beyond a slightly improved incremental change in a small part of the audio market. AP is MUCH bigger than that.
Another argument for selling is simply that you need the money for other things. That may apply to the likes of you and me, but I doubt it has the same pressure on the Bart group or the other major holders.
(20min delay)