I have a few comments having caught up on this thread.
See and avoid.
It is a nice idea. However, there have been a number of studies that suggest at best it works poorly. I have posted before the times we typically have to spot another - they are quite sobering, particularly if you factor in our reaction time (which we usually forget about).
The truth is the majority of the time collision avoidance is down to "good luck".
I am sure we can all recall on a busy days when we have been receiving traffic how little of the reported traffic we actually see.
Avoidance.
There are a number of good strategies to reduce the risk of collision. The usual suspects are to fly as high as possible (there is far less traffic), to think about choke points caused by CAS, navigation or terrain, and, best of all, leave the landing light on in conditions of poor visibility.
PCAS.
I am pretty much convinced the system works well most of the time. The more basic of the units is still very useful even though it does not give you the direction of the traffic. The key here is that it will warn you if traffic enters a "protected bubble" around your aircraft. It displays the vertical separation. If traffic enters this bubble you climb or descend to ensure a vertical separation is maintained.
The systems provides an audible warning so there is no need alter your head out of the cockpit time.
All in all, keeping a good look out can do no harm, with luck you might avoid an aircraft by doing so, PCAS adds, for a very small cost, an additional (and in my opinion significant) margin of safety which combined with defensive flying and receiving a RIS whenever possible all help to reduce the risk of a collision significantly.
Should all aircraft carry a transponders?
I was for many years opposed. It is important to protect our rights and to recognise the problems fitting a transponder to some aircraft and gliders. As the cost of these units has fallen I find it more difficult to justify exempting some aircraft, whilst appreciating that in some cases cost is not the only factor.
I would guess that if there was a market a portable transponder that only squawks 7000 would be cheap to produce and could be made so it was both portable and could be powered by a self contained NiCad. If the purpose of a transponder in the more "basic" aircraft is only to enable the transponder to be interrogated for collision avoidance and the pilot has no intention of operating within CAS this would seem a very simple solution.