mostly because I want to replace as many of the bulbs that come on when you turn the parking lights on, so I can use my parking lights as running lights in times of lower visibilty.
Been there, done that!
However, I have noticed that I still get slightly better FE with the lights fully off (i.e. no light electrical load), than I get with just the (now mostly LED) "running lights" on. However, the LEDs do greatly lower the FE loss of turning on those secondary lights (from what the "hit" was when those were incandescent bulbs). So even though I still get some FE hit, it's now small enough of a hit that I'll turn those (secondary) lights on whenever I think they will make me significantly "easier to see" by other drivers. i.e. I'm willing to take a very small FE hit, in order to use my secondary lights to improve safety.
If you are really being a "fanatic" about this, don't forget the "smaller" lights, as the watts on them can add up. For example, their 5-LED "WLED" (small "wedge") units seem to work very good to light up my license plate (thereby saving aprox 8 watts vs using two incandescent bulbs to light up the plate). And I also used those small wedge units for the car's "indicator"/"running" lights (saving a lot of watts in the process). And I think I saved another 12 watts or so, by using "74" and "T1.5" modules to convert my dash backlighting to LEDs (and got a dash that was easier to see, with less glare at night, as a nice bonus). And while I didn't measure how much power I saved by cutting the "back light" wire to my radio (thereby preventing the radio from "lighting up" when the car lights are on, but still letting the radio otherwise work normally), I'm sure it was at least several watts. And as an added bonus for cutting that radio (backlight) wire, I no longer have the radio's back light interfering with my night vision while driving (so I actually PREFER the radio this way, as it stays dark at night, except if/when you turn it on to play something)...