Originally Posted by Wallace
In a less snarky tone: "rolling resistance" has been shown over and over to be a complete non-issue in competitive cycling--it's all about weight. Compare the weight of a sew-up on a carbon fiber rim to any clincher and you'll see why the pros only ride tubulars.
Having said that, tyre weight and rolling resistance are roughly
Performance of a tyre is a function of many things.
When speed is of primary concern (over say durability, handling, puncture resistance, comfort, cost, ease of use/replacement), then the tyre's rolling resistance (Crr) and its aerodynamics (CdA) are the primary considerations.
Crr is a function of many characteristics, such as tube type, threads per inch, weight, rubber compound, construction, shape, width/profile, and if it's a tubular, how it is glued and what it is glued with. Inflation pressure is an issue as well (and one that's often misunderstood).
Cda is dependent mostly on tyre's width but is also dependent on the rim you choose to ride it with.
Often there is a trade off between the two, and the best rolling tyres may not be the most aerodynamic, and vice versa.
Tyre weight is of little significance to performance (beyond how it relates to Crr). 2 orders of magnitude less important than aero and Crr.
Besides, Pros ride what they are given by the sponsors.