They probably don't include the second mount default because they assume the majority of people are only going to be using it on a single bike. Those that are going to be swapping it between two bikes are likely going to be willing to dish out the $10 (I think that is a little more than most mounts go for anyway) for the second mount, rather than increasing the price on the whole unit by several dollars for everyone including those who would only use one unit. It is the same thing with so many other electronics. The device has X different capabilities, but only includes the equipment for some of those. The remaining capabilities require additional add-ons. They generate additional revenue for the company and reduce the cost for baseline users.