Intel probably cared too much about power consumption for Haswell. That's probably the main reason why clock speeds for Haswell have remained so low.
There's a lot of truth to that, although the matter of it being "too much" is subjective, of course. If desktop CPUs are the only thing one cares about... yeah, it's can be tough to swallow.
The IVRs are undoubtedly the primary reason behind Haswell's higher TDPs compared to many of the IVB SKUs. IVB has a TDP of 77W, while Haswell has a TDP of 84W. Real world power consumption, even at the system level where a lot of the system board level VR circuitry is reduced, is generally is higher under heavy load as well.
Even assuming a
90% efficiency, this already puts Ivy Bridge at Haswell's power level (77W / 0.90 = 85.5). TDPs are coarsely related to power consumption of course, but the effect can be seen clearly. The Hillsboro team must have done a lot of work to keep power from being higher than it ended up being, despite the integration of the VR, especially when you consider the expansion of the GPU.
Then you factor in that the IVRs are nestled up against the "long" sides of each core:
...and there goes your overclockability.
The implications for mobile however are very good.
Board size goes down significantly (PDF download warning), as does the bill of materials by about $5 (which can be applied to desktop boards as well). And as I'm sure you know, battery life can be as much as 50% higher with Haswell. The U-series and Y-series SKUs benefit most from this.
There are benefits that the IVR brings to overclocking, like the extremely clean power being supplied, but they're significantly outweighed by the fact that there's more power and heat being released, and at close proximity to the CPU cores. Unless these can be built with near 100% efficiency... these IVRs are probably always going to hold back max overclocks. Still, there is a long way to go in the realm of integrating power supply components on die.
However, in my opinion, it was a change for the better, overall. I don't know the whole story behind Skylake's exclusion of the IVR, but from everything I've seen from Haswell, it wasn't the right decision. It'll be awesome when Atom gets inevitably gets FIVR, just like it's awesome for today's Haswell-powered mobile devices.
That's the thing - 49 W for idle is way too much. I'm sure there's more Intel could do to improve on that, but surely the rest of the system would have to be improved to get that number down.
A lot of that is dependent on the motherboard being used. I highly doubt that the majority of that 49W is from the CPU. In fact, it's the dGPU at fault, if he's got one installed.
Normally Haswell systems will idle in the 20W range with a decent board.
The processor can actually get to 45mw in C10. Even AMD's previous generation Richland processor
idled at ~1.6W. Haswell should be well south of that. In fact, C6/C7 calls for a minimum 0.05A of power on the 12V2 rail, which is 600 mW. Even Ivy Bridge had a C7-state TDP as low as 2.2W. I think it's pretty clear that it's not Intel's hardware that's responsible for the 49W idle.