TNR is a touch subject, with many variables. Some people feel that the feral cat colony should be euthanized entirely, that they wreak havoc on the natural ecosystem by killing the birds and lizards for food. Some groups test for FeLV & FIV and euthanize those that test positive, while others test but don't euthanize unless symptomatic.
The standard for neutered/vaccinated cats is for their left ear to be "tipped", so the same cat does not go through the trauma of getting trapped and taken to a facility after already being neutered.
I previously worked for a SPCA Spay & Neuter Clinic, where we handled feral cats routinely. Most of the feral cats were FeLV & FIV negative, and otherwise healthy. I live in an area where feral cats are plenty, and would LOVE to see better TNR programs around us!
FYI- National Feral Cat Day is October 16th!
Here are some Feral Cat links-
Alley Cat Allies is an amazing organization promoting TNR and awareness of Feral Cats. I would love to become more involved with them someday.