I thought maybe I should jump in and do my best to clarify a few things from Mavic's perspective about the R-Sys Wheel Tire System, and the WTS concept in general. Forgive me if I sound too corporate or polished, but a little context could help explain a lot.
First about the R-Sys specifically, including stiffness and rotational inertia. The R-Sys was designed to be a unique, innovative, all-around wheel solution for the large majority of cyclists who like to ride long distances in hilly terrain. Gran Fondo/Sportive style.
Starting in 1999 with the Ksyrium wheel system, Mavic pioneered innovative materials and construction methods to make wheels lighter, stiffer, and stronger, yet still maintain functionality and serviceability. So for example, the Zicral aluminum spokes, the FORE spoke insertion drilling, and the dedicated hub shell/lacing pattern. Example: Zicral alloy is less "stretchy" than stainless steel, so using this material for the spokes adds a lot of stiffness. Another example: by using the FORE drilling process, the tire bed in the rim is not perforated, therefore the structural integrity of the rim itself is preserved. Again, stronger/stiffer. Taken together as a wheel system, the Ksyrium wheels set a new benchmark for low inertia and stiffness. The result was wheels that people enjoyed riding because they were light, stiff, and pretty darn tough.
The operative phrase there is "wheels that people enjoyed riding." You can certainly make a case that wheels with low inertia (low rotational mass) offer substantively little benefit from the standpoint of pure physics. Especially climbing or riding at a steady tempo. On the other hand, braking, cornering, and acceleration are all significantly improved by reducing rotational weight. And from the rider's perspective, there's a benefit in terms of sensation - they just feel good to ride because they snap up to speed and carve corners nicely.
Enter the R-Sys. It was made almost to be like a next-gen Ksyrium. The carbon spokes are lighter. Plus, they're un-stretchable and un-compressible, so stiffness is through the roof. The spokes don't need a lot of tension, so the anchor points (hub shell and rim) suffer much lower stress cycling, and can therefore be made lighter. Also thanks to lower tension, bracing angles can be made greater (wider hub flanges) - again, more stiffness.
There are a lot of unique solutions to the engineering challenge of "bicycle wheel" in the R-Sys. The result is once more: lighter, stiffer, stronger. Aerodynamics was less of a consideration, but when actually tested in a bike they're not quite as bad as you'd think.
So what you get are wheels that are super nice to ride for long days in the mountains. They're light, durable, they handle well, and they feel good. The SLR version this year is below 1300gm for the pair, which given their stiffness and strength is pretty sweet.
Now regarding the tires.
The reasons for Wheel Tire Systems are several. First and foremost, over the course of testing many, many wheels (always with tires in place, as required), Mavic discovered significant variability in tire quality. Not just normal stuff like compound and casing, but precision of fitment, roundness, clincher bead stretch, and the like. Tolerance issues.
By adding tires to the system, Mavic can control tolerance on this critical aspect of the performance package. Not just grip and shape, but precision of fitment and tolerance for bead diameter, stretch, etc. These are important safety considerations, and Mavic wants to control for them in addition to fundamental performance criteria like weight, grip, puncture resistance and the like.
Aerodynamics were first in the formula for wheels with the CX01 technology (including CXR 80 and more to come ..). With R-Sys and the others, grip and weight were weighted more significantly in the design. The tires on the 2013 WTS wheels mount fairly narrow, so I bet they test better but any gain would be marginal.
Now that I wrote a dang book, please hit me back with questions, comments, or anything else. in public or PM. I'm getting a thicker skin so I won't take criticism personally.