I shoot an EX-1 with Redrock M2 and the flip adapter on a regular basis. We use a set of mostly Nikkor primes (35, 55, 85, 135) with a few cheapish Sigma lenses thrown into the mix. Most of this is for higher end corporate video, so we don't get on the Sigmas very often (a zoom, a fish eye 15, a 24, and a 500 mirror).
It works great on the EX-1, although I find Red Rock's follow focus leaves much to be desired. The internal gearing has tons of slop, the "mark" on the follow wheel is like 1/8" (maybe 1/16" or between) wide - too wide for spot on marks. All of this exacerbates the problems of pulling focus on still camera lenses.
Regarding the EX-3, you get the advantage of being able to mount nicer glass directly on the camera. What you don't get is the DoF effects of using the 35mm film plane inside the adapter. Those two cameras use the same sensor setup: 1/2" image planes. This gives both cameras a DoF more like a video camera unless an adapter is used. It *is* possible to get mount rings for the EX-3, even ones for older Nikkor lenses, but there's no real change to DoF characteristics (unless they are spacing them out farther from the sensor in ways I am not aware) since you are still on the smaller 1/2" senors.
What you get out of these adapters is a lens in front of a 35mm plane which generates an image that acts the same as a 35mm camera in terms of focus. You then capture this image with the camera behind the adapter.
The real question is what else you plan to do with the gear, and what your clients can afford in terms of kit rental. For example. We do shoots both with and without the RedRock - because using it costs more to the client so clients only want it when they are willing to pay more for the aesthetic effect. My producer recently bought a second camera, and in deciding between the 1 and the 3, he chose the 1 because pricing out the 3 correctly would limit what clients would be willing to rent it, among other reasons.
So, if you've got folks willing to drop the extra kit rental for the more expensive camera (without the adapter and lenses, remember those cost extra on top of the base kit) then go for it. If you want to service clients with smaller budgets, go for the 1 and retain that market.
Sorry for the long response. The answer to your basic question is that yes, there is a reason to drop an adapter in front of an ex-3.
