I had a discussion with Bill Moffit from Navworx about this. According to Bill, the way the FAA is operating the system, you will need to have ADS-B out to ensure you will receive traffic near you. Each ADS-B ground station controls a very large volume of airspace. FAA is not broadcasting all of the traffic in that airspace, they are only broadcasting the traffic within a small cylinder around those aircraft they receive a transmission from (something like +/- 3500 ft and 7 mile radius from the ADS-B out equipped aircraft).
So if there is only one target in the volume transmiting via ADS-B out, and he is far away from you, you will not receive any meaningful traffic info via ADS-B in. If he is near you, you WILL receive some relevant traffic info depending on how far he is. To me, the more appropriate term for this might be NARROWCAST rather than BROADCAST. Weather info is truly broadcast, so according to Bill, the only reason to buy the RECEIVER ONLY is to get weather, you need the TRANSCEIVER to get meaningful traffic info.
Oh and by the way, since ADS-B has GROUND stations rather than satellites, you need to be airborne in most places to receive the signal. No checking weather on the ground prior to launching on that long X-country and maybe no traffic at pattern altitude and below in many places. At least until all aircraft are equipped and aircraft to aircraft transmissions become the norm.