The fact is that one CANNOT rely on the Mk1 eyeball. If you want to use stats to prove this, how many collisions have occured in VMC conditions? Pretty much ALL of them
AFAIK.
TIS is a real benefit, and I fly aeroplanes with it around LA which is exceedingly busy as no doubt Socal will atest to, and I have had a number of close calls in the past over the famous Long Beach "practice area" - which has claimed numerous lives in recent history from midairs.
If anything TIS increases ones ability to visually acquire a target - rather than "keeping your head in the cockpit looking at a screen" as some people who probably have never used the system seem to imply. Also because you can see the altitude of conflicting traffic you know if there really is a risk or not. I had a controller clear an aircraft for a left hand departure from a right hand parallel runway just after I had taken off the left hand runway at night. The other aircraft passed overhead at less than 100' and it was only because TIS started screaming that we stopped our climb and put the nose down. being night it was very difficult to tell which way the aeroplane was actually moving until very close when things started to happen very quickly.
However, one thing that I have discovered is that luckily a midair is exceedingly unlikely. Even when your traffic system shouts "Traffic" at you most of the time there will be some offset in altitude - A miss is as good as a mile so even with 20' clearance, a collision won't occur. Still sods law and all that, and so if I see another target I have not visually acquired then I'll make sure there is some offset in altitude.
In the UK i have a Zaon XRX interfaced to my G496 which displays the traffic on it. It works well and certainly increases situational awareness.