Chilli
I sympathise with most of what you're saying but I do think the specific example you gave is a bit OTT.
A VFR aircraft is asked (or told: was it class D or class G?) to maintain separation from another aircraft which it has reported to have in sight. According to your narrative, it does so without intervention from you, and without the other aircraft changing its flightpath.
What's the big deal here? Does the carriage of TCAS in this day and age mean that standard separation must be maintained even when aircraft can avoid each other visually, just to avoid setting the bells and whistles off -- I don't think that's the intention of it.