Old pilot is right. But keep in mind there are lots of 1090es aircraft like RVs and many other certified planes that elect to add the ADSB Out "ES or Extended Squitter" to their Mode S transponder like a Garmin 330, Trig, etc. These ADSB Out planes would be using the 1090es and they would not necessarily be "high altitude traffic" as many have service ceilings well below FL180. In my opinion, the FAA and some manufacturers messed this up for us.
With that said I think the 1090es "In" is very important for seeing traffic when more distant from the ground stations and especially at low altitudes. Remember if either you or the traffic is at a lower altitude, under 1400 AGL, the range from the ground station is reduce greatly. To get the maximum traffic, you'll want to be able to receive both 978 and 1090ES. I hope this helps.