Is this based on fact, guessing, or repetition of rhetoric you've heard from others? The only reason I ask is that in my personal experience, the difference made by lighter weight wheels is quite drastic.
Bike A) approximately 20.5 lbs. Ultegra 9-speed, average weight bars, stem, seat, seatpost, etc. Heavy wheels (approx 2200grams w/o tires, tubes or cassette). Extra weight accounted for primarily in the wheels and and secondarily in the frame.
Bike B) 18.5lbs. Ultegra 10-speed, average weight bars, stem, seat and seatpost. Comperable-weight tires & tubes. Approximately 1850gram wheels.
Bike A was all I was previously used to and figured that climbing had to be that difficult. Purchased Bike B and proceeded to ride both around my hilly neighborhood, quickly jumping off of Bike B to Bike A and then back again. The difference climbing a small grade, sprinting out of the saddle, etc., was simply enormous. The frames are both very very stiff, but both bikes performed entirely different. Bike B with the lighter overall weight (most of which was accounted for in the wheels) made Bike A feel like a sluggish 4-cylinder truck with a full load of firewood in the back while pulling a small camping trailer.
I proceeded last Friday to ride Bike B on my normal route that I'd taken only Bike A on previously. The weather was cold, I felt physically slightly sub-par generally speaking, and it was a slightly windy day. Not "blustery" per se, but definitely not the normal environment I'd previously been used to riding Bike A in. By the end of the ride, I'd averaged approximately 1 mph faster than I'd ever ridden this exact same route before - and THAT was with me lolly-gagging for a solid 6-7 minutes at 10mph for a stretch while watching scenery that I'd never taken time to slow down and enjoy before.
My personal experience leaves no doubt whatsoever in my mind that the weight (especially rotational) weight makes a huge difference.