Actually, in most cases I think they're just overestimating the magnitude of the problem. So I think you're both wrong, really. It definitely DOES make a difference. It's just a matter of how much of a difference.

On a road bike you can really feel it if you go from lightweight climbing wheels to a deep section aero wheel. The climbing wheels just feel a lot snappier. But it's fairly subtle. It's not a matter of "I can climb that on climbing wheels but not on aero wheels", just a subtle "the bike feels a little more nimble on the climbing wheels".

That being said, the pro road racers use a wide variety of wheels - not just the super light climbing wheels. Why - well, that goes back to the old "it depends". It depends on the terrain, what sort of race it is, etc. If you're on a flat time trial, not only does the aero advantage of a full-disc wheel outweigh the weight penalty, but once you get that full disc up to speed it has it's own momentum helping to keep it up to speed and roll over undulations in the road. Then theirs the gear issue. I have about the lowest granny available on my bike and I need all the help I can get!


Now on an XC mountain bike it's a little more complicated but the same principles apply. Yes, the 29" wheel is going to be a little heavier and that weight is going to be a little further from the hub. But you still have big balloon tires, big knobs, etc. So either way you have quite a bit of "mush" in the system. Then you're going over rocks, ruts, logs, etc where a heavier wheel might be more desirable for rolling over and through that stuff - just like a TT disc can be desirable on an undulating TT course. So in some situations the 26" might be desirable, in some the 29".

I don't get how the bigger footprint is a disadvantage for the 29" wheel - in a lot of situations that's going to be an advantage - more tread on the ground = more traction. Plus footprint is going to be impacted a lot more by the type of tire you run and how much pressure.

I'm not sold on 29" wheels by any stretch. I don't see them being much of an advantage and probably the advantages and disadvantages balance out. I'm fine with my 26" wheels. But the bottom line is I just don't think it makes a heck of a lot of difference.