I have a 27-inch cabinet that houses a microwave without a trim kit. I want to replace it with a new SS finished microwave. The micro I'm thinking of using is a Sharp countertop model (R-51ZS) which has an optional trim kit with vents at the top and bottom (RK48S27).

I don't want to use the trim kit. My cabinet is finished on the interior to match the cabinets and I'd rather just have the micro sitting inside, shelf-style. The opening on the cabinet face frame is 24 inches wide with an inside dimension of 25 1/2 inches wide. The microwave itself measures 23 inches wide.

Problem is, the manual says to allow four inches of clearance at the top and sides. I have plenty of space at the top and also at the back, but only 1 1/4 inch on each side. Yet, these clearances are not required according to the trim kit instructions. As far as I can tell, other than the venting slots, there doesn't appear to be anything in the trim kit that would force the air away from the sides and out the top and bottom vents. The trim kit does include a bottom assembly that lifts the microwave off the bottom of the cabinet a bit, but it looks like that could be easily accomplished by other means (if even necessary at all if not using the trim kit).

Is the trim kit really necessary? With no means of forcing the air out the vents, is it really doing anything functional?