It absolutely needs a solar sensor for the ET calculation and of course it would need the other two sensors for the soil temp and leaf wetness reports. I guess if you didn't have those sensors, the report capabilities would be restricted.
I'm mostly interested in its ET capabilities. The standard WL ET output is pretty limited if say you wish to know the total ET over certain periods of time that are not simply a monthly or daily interval. Sure you can add up the daily bar values when you set the plots to a weekly range, but that is a pain to do. It is supposed to sum total ETs over your chosen time interval.