Clutchless downshifting very useful in dirt riding as a technique to get the back tire loose going into a corner. So if you are riding your Ninjette on dirt and trying to lose the back tire, go for it.
Otherwise, use the clutch on downshifts. You want smooth downshifts on the street to keep engine braking and thus your rear tire under clear control. Using the clutch with a blipped throttle is great for smooth downshifts.
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"Improvement makes straight roads, but the crooked roads without improvement, are roads of genius." — William Blake