Akro, (warning: rambling content
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The best use for ADSB is for the areas outside RADAR and to provide better update rates than a RADAR system. Especially valuable for trans Oceanic flight tracking.
While ADSB-In has been touted to provide all sorts of wonderful benefits, it will likely never, ever happen. As noted in the Boeing document, they fully support ADSB-out, and say they will look to meet mandates for In. The plans show for ADSB-In to be used on the ground, even equipping all ground vehicles. Nice diagrams and work for the next 30 years, but I am sure that technology will run right past the capability. The ground movement has pretty much stalled because ADSB is line of sight, and when the ac is sitting or on taxi, the runway surface reflection, other aircraft, and structures, blocks the signal.
All of this ADSB has been in the works for many, many years, operating in a silo, and not keeping up with technology. Its broadcast is limited to line of sight, low power, so in AUS, while you may be broadcasting, there would have to be stations to receive, with GA altitudes, a lot of stations!
There are far better, faster ways of achieving the same goals with current and near current technologies. That is why I feel it will languish like MLS.
The significant reason why you will never see any commercial system rely on ADSB data is that the signal is not secure, and there is no way to validate. It is very easy to spoof the ADSB broadcast signal, so you really dont want to turn on the 'In' of your FMS. ADS-C is a contract, and does have some security capabilities.
What kills me is the 1090 band is already used up in many areas of the world, and the capabilities havent even been turned on yet. So, now they are looking at ac below 18,000 to use 978. Lets back up to the first point. The FAA is already aware the system is busted with just the basic information being transmitted. The 'congestion' as they call it, means it starts leaving aircraft out of the system