I agree to a certain degree. I use the Iray preview a lot when I am positioning skydomes, HDRs, and large structures... However I try not to do major movements with it turned on.

I use the Iray preview when I have a fully populated scene; which happens often towards the end of a project, In these cases I will turn it on, view the results, then turn it back to Shader rendered. Make changes, then turn it back on.

Even with the dual 20x0 GPUs, moving objects around can be pretty choppy in Iray preview. (I think that Iray preview will only use the active GPU, but I don't know for sure.)

I was really never happy with the 3DL results I would get, I just wasn't skilled enough to harness it. So I am much happier in the Iray days.