Well integrated graphics on the desktop are still primarily a cost cutting solution. You don't have the issue of battery life there. I've always suspected at some point people who use their notebooks in public will no longer have to locate power outlets for extended periods of use. Looking at the chips that incorporate iris pro graphics, it's unlikely that the cost savings is significant over some of the less expensive discrete chips. I can't say for sure as I haven't been able to locate a cost estimate for something like a 650m.
I remember intel some speculation that ram clock frequencies weren't quite there yet. That might explain the additional cache on some chips as opposed to a longer evolutionary path.
This is a problem that both AMD and Intel have with their APUs. I wouldn't be surprised to find dedicated RAM for the GPU right in the SoC in the future. Right now packaging in a separate RAM die is a compromise to get around the bandwidth to RAM problem. It should also be noted that Intels focus with Iris Pro is on mobile where you don't have the option of wider paths to memory.