MK1 eyeball is so much better and even if you have, like in the aircraft I fly, TCAS 2 fitted when flying VFR you still should spend most of your time heads outside scanning for other traffic.
True, but the Mk 1 eyeball also has well documented limitations.
Additionally, it is totally useless if another aircraft approaches yours from an blind spot such as above (or below) and behind you.
Unless you have a 360 degree view from the cockpit and a neck like an owl.
TCAS is a very useful supplement to help build a pilot's awareness of what is going on around him. As I wrote a few posts back, it needs to be used correctly, with it's own limitations borne in mind.
Same as listening out on the radio. Another useful tool, but no good in isolation.