Nobody would deny that the Mark I Eyeball has its limitations. Getting a little of the right kind of assistance is no bad thing.
HOWEVER, TCAS/PCAS/OCAS also have their limitations and cannot under any circumstances replace a bloody good lookout. They can't show anything not transponding, they can't show anything if the equipment fails, and as Pace so rightly says, they CAN very easily induce a false sense of security so that the lookout becomes lazier and less effective.
I train in an area that can on occasions become overrun with gliders, which very rarely carry transponders and often are non-radio as well. They don't reflect primary radar too well, so are all but invisible on radar, invisible on TCAS, and you need a damn good lookout to see such a slim fuselage and high aspect ratio wing.
So keep that lookout skill honed - or you will become (the cause of) another accident statistic sooner or later.