The Xeon 'L' parts (as well as the Core CPU 'S'/'T' parts) only have a lower TDP because they've had their top-end limited. At idle they're really no different than the mainstream CPUs. And at load they're less effective (i.e., slower). Also, they're often more expensive.
Intel's Low-TDP variants are intended for applications where thermal performance is limited, such as small/tight cases with little airflow. In a regular case they're wasted potential.
There is truth to the thought that Intels chips all have very good idle and they are not far apart when idling. However, the L chips are also heavily binned and have lower vcore by default than non-L chips. Yes you could probably undervolt and underclock your i5 and get to a similar place...Hold that thought for a moment, because I really do want to get to the bottom of this once and for all. When I compare e3-1220L v3 (ARK | Intel Xeon Processor E3-1220L v3 (4M Cache, 1.10 GHz) ) with e3-1220 V3 (ARK | Intel Xeon Processor E3-1220 v3 (8M Cache, 3.10 GHz) they look completely different. The e3-1220Lv3 (13 watts TDP) has half the cores and 1/3 the base frequency of the e3-1220v3 (80 watts TDP). Yet you're saying that at idle they're really no different? At least on the face of it, that just doesn't sound right.
Frankly 1/6 *80w gets you 13w ... so these V3 chips scale pretty darn well.
As I have a 1.6ghz 1220v3 as well as quite a few other v3 chips ... I could do some testing in the coming weeks.
Suggestions as to accurate power meters?