Hi Brendan, we had a club Tecnam make a forced landing here some time ago. Very experienced pilot, long descent to finals, I think, started running horribly rough so he set it down. Started and ran no problems thereafter, no fault found and subsequently returned faultlessly to club use, so high likelihood it was icing.
The aircraft I mentioned on short finals was a Savannah, they take air from outside the cowl, the pilot described it as being like the engine wanted to leave the aircraft.
Selecting carb heat in my Savannah certainly results in reduced power, I wouldn't call it negligible.
As for the kiss principal, you are in no way complicating anything by having heating available, as most of the time you won't be using it. So I don't see the advantage of kiss here. What it is is an option that's very nice to have on the day that your engine starts to cough and run rough...