Ok here are three (3) examples from the last two days to show why I think radios are imperative.
Yesterday afternoon flying from Toowoomba to Tara (I know Toowoomba is mandatory radio but I could have departed from one of the three strips that I know of that are not) Qlink 18D inbound to Wellcamp tracking north west for the 12 RNAV Z which placed us on perpendicular tracks within 2 minutes of each other, with some commonsense radio work we were able to maintain separation standards as applicable for the company policies relating to the Qlink operation without either of us diverting from our planned tracks. Without the radio I wouldn't have known he was there and vice versa.
This morning departing from our private strip taxi call (as I always do, not required) and back on the radio was a helicopter 3 miles to the west tracking east directly overhead, I had no idea he was there (no need for him to make a call not required) could have resulted in me climbing straight up into his path.
This morning into Toowoomba 3 King Airs inbound number 1 estimating 1 minute before me, number 2, 2 minutes after me and number 3, 3 minutes after that, we slotted in with each other without delay to any of us also 2 King Airs departing around the same time, once again sensible radio and separation stragities used, coms with both Brisbane centre and the aircraft involved, couldn't have done it by see and avoid.
Other things to consider you can legally fly up to 5000 feet without radio what happens on your descent when you are flying through other levels with the sun in front of you without radio you have very little chance of seeing another aircraft on a reciprocal track.
Aldo