I detect the same prejudices in some quarters here as happens with GPS.
TCAS allegedly damages your lookout.
GPS allegedly damages your traditional navigation skills.
Why not just have a long spike in the middle of a steering wheel, and ban the use of seat belts. It would do wonders for road safety - without a shadow of doubt. Mandate crossply tyres, with excellent aquaplaning properties, for good measure.
But seriously, in the GA OCAS context, one uses TCAS to assist in visually spotting the target, and if one cannot visually spot it then one has no option but to take avoiding action based on TCAS alone. It is better than doing nothing.