Just a sales gimmick. On some older cars, that have carbon buildup in the induction system caused by abnormal operating conditions, it could make sense, but to do it "occasionally", no, unless it says to in the owner's manual, which is highly unlikely. I would love to see
you pin this person down in a constructive but relentless way and make him explain exactly why this has to be done if it's not mentioned in the owner's manual. If he says it's because you may get deposits inside the intake manifold or something like that, ask him why that should happen on a quality make like Toyota that's well maintained. Please post back.
Of course, I understand you not wanting to get in a stressful situation vis-a-vis you and the dealer, but it boils my blood when I hear about these sales gimmicks. Once in a while, they may be legit, but hardly ever.