SSD, I agree with half of your post.
In flight: this is the last place yoiu should be reading a checklist
I agree. In flight, I always memorize check-lists. There are a few exceptions - rarely used check-lists which aren't critical, and aren't needed at critical stages of flight. For example, when flying an Aztec, I was taught to memorize the actions to take in case of engine failure, up to the point of getting the engine feathered. Once the aircraft is under control and the engine featered, I'd reach for the check-list and read out the engine re-start checks. But in general, in-flight checks in every aircraft I've ever flown (and I suspect all single-pilot aircraft) should be memorized - certainly the downwind checks should be.
But on the ground, I don't see any reason why you shouldn't follow a check-list to ensure that items aren't missed. What if the panel of your aircraft is laid out such that left-to-right won't work? What if the fuel selector is to the right of the fuel pump, for example? You'd need to make an exception - work left to right, except remember to turn the fuel on first. How many exceptions are acceptable before you need to use a checklist? Why not just use a checklist anyway? I can't see any reason not to use a check-list. When I was training, I constantly forgot to turn my mobile phone off before taking off - so I wrote it on the top of my check-list! Working across the panel from left to right would certainly never include checking my mobile phone is off - but my check-list did.
Externals is an interesting one though - I personally prefer not to use a check-list for externals, because the check-list can't possibly cover everything, and by following a check-list you're more likely to miss something. Much better to keep your eyes on the aircraft, keep them moving over the surface looking for anything which seems out of place, whether it's on the check-list or not. But I can definitely see that there's a good argument both ways for externals.
As for not doing inappropriate checks, as I said earlier on the thread, I disagree - learn one check-list, and use it for every aircraft, even if there are inappropriate checks. I've flown aircraft with fixed and retractable gear, with and without carb-heat, with and without a mixture control, with and without a fuel pump to turn on for go-around, with and without hatches or doors - but I always check all of those items down-wind. I've never yet forgotten to check an item on one aircraft, just because I didn't need to check it on the previous aircraft I flew, and I'd have thought you'd be more likely to do this by trying to memorize different check-lists for each aircraft.
FFF
-------------