the lack of Mode C may increase controller workload
ffb - it does increase workload as we may be calling traffic that is not relevant. I have of late taken to asking pilots I'm providing a sevice to if they have mode C - you would be suprised how many have diliberately not switched it on.
As you rightly mention those who think they are getting a radar serivce because traffic information has been passed (under 'duty of care') are the problem. If we could give a true FIS and not have to keep an eye on the traffic to advise about conflictions we would have more time to provide RIS and RAS.
PM