Try 3.5 watts, not counting overhead of most USB power bricks being incredibly inefficient.
A current-gen 35W laptop CPU will be some 10 times faster as a RasPi, have much faster storage available (SATA3 or NVMe versus… USB2), much faster I/O (GBit LAN and GBit Wifi versus… USB2), and a lot of other benefits. (Like an integrated screen and battery and keyboard and …) It also won't need external hardware to communicate with other cluster members – that 10-port ethernet switch will need power, too.
One RasPi is relatively energy efficient; RasPi clusters… not so much.
> But really, I think the primary use case is cost.
 http://raspi.tv/2016/how-much-power-does-raspberry-pi3b-use-... , see the numbers for "Multi-threaded CPU Tests", which is the most applicable for server workloads
 Running that script manages ~9 runs/second on an i7-6700HQ, vs. ~0.9 run/second on a RPi3.
And at $5 each, if we're talking hardware costs for setting up a "toy" cluster for, say, self-learning or student labs, that's hard to beat. I suppose you could do better using VMs for a virtual cluster, but that adds other complications unrelated to the clustering task. But I agree there doesn't otherwise seem to be much practical purpose here, and the overhead of running an OS on each Pi really cuts into performance compared to a single chip w/ multicores instead.
At the same time, you're comparing the power consumption of lets say 10 whole RPis/platforms to the consumption of a single processor. Stick that processor in a platform (laptop), and it's going to use much more than 40W.
Like you said, you get a lot more with the laptop, but given your benchmark (10x difference), my guess is that 10x RPis would still be more power efficient than a laptop with a 6700HQ at that specific task.
Gets you 11 Pi's ...
Gets you only 1 Intel CPU, no memory, motherboard, heatsink, fans.
Reminds me of the Celeron® Processor J3455... 10W rating on Intel there page. On AVERAGE! Then when you see the real power usage under load for MB + CPU + 16GB memory, its actually doing 35W.
Where as the Pi's are doing 3.7W max per piece. So even with 4 pieces to match the performance, your still half the wattage.
If Intel really scaled that good in power vs performance, why are we not seeing x86 phones all the time?