At one airfield I fly at the ATCOs are insistent that we turn off the tx in the circuit. They don't want it cluttering up the screen in (for them) an unnecessary area of their airspace.
I don't want to prejudge the report on the Colorado collision, but I wouldn't mind betting that the Cirrus pilot might have been head-down in the cockpit and not looking out, whilst the glider/tug combination with limited maneouvreability might not have alerted the Cirrus's TCAS.
I have had it drilled into me since early flying days - use all the technological aids you are able to but don't put your faith in them not to let you down when most needed. Always be aware of limitations.
For example we've just had an AD on our Narco Mode C which has been found to give spurious readings.