I know enough physics to understand the significance of inertia, moment of inertia, and acceleration. So I would dispute that extra mass, especially on the wheels, matters only in a climb. Don't know about anyone else but my speed is rarely constant for very long, for all kinds of reasons. Braking requires no effort, but every time I have to accelerate back up to a preferred cruising speed I feel the bike's and my inertia fighting back. It wouldn't surprise me to see experimental data (which I don't have) that shows my speed fluctuating up and down a tiny bit even within a nominally flat, constant-speed section. Since I don't race I'm not too concerned, but it does make a difference.
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The ultimate point being: Lighter wheels let me think I'm going faster. Self-delusion is the only way I'll ever really be fast, so I intend to continue the delusion.
