AIUI the Vigilant is operated with the carb heat to hot for all but take off and landing due to the supposed aforementioned risk of carb ice.
That in itself will cause issues associated with running rich - rough running, wash oil off the bores and contaminate the engine oil, it has nothing to do with '12 minute circuits' (and just how big is a 12 minute circuit?).
Regular carb heat checks are taught elsewhere as a precaution against carb ice and if taught properly and with a certain methodology will not see carb ice being an issue (think O-200 carb ice problems, the CAA say it's the worst ice maker - if you adhere to regular checks it's not an issue).
Having flown a 109B on a day that was more conducive to carb ice than most I wouldn't say it has a tendency to ice up any more than, say, a C150.
And as an illustration of the lack of general aircraft knowledge and operation, I know of some of some Air Cadet instructors who insisted upon operating a privately owned aircraft with the carb heat hot all the time.
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