I was debating not participating in this thread, but what the heck...
I mostly agree with
@EdFred. The regulation SHOULD say "flight time is time that accrues when the aircraft is airborne" or something like that. The current definition of "moves under its own power for the purpose of flight" is unnecessarily vague, as it lends itself to this whole discussion. Personally, I think that years ago when the wording was first written, "they" PROBABLY meant "beginning takeoff roll" as "moves for the purpose of flight". But I have nothing to support that, AND there have been subsequent letters to the contrary. Doesn't mean I agree with them though.
I just base that off a common-sense definition. If you asked any non-pilot what they thought "flight time" meant, I'm willing to guess almost all would say "well, duh, it's when the airplane is in the air". Which makes the most sense to me.
The argument is occasionally brought up that taxi time SHOULD count for "flight time" because you are doing PIC-responsible things and have important flight duties to carry out, etc. I don't buy this argument - you do PIC-responsible important flight duties before ever getting in the airplane, too, but obviously those don't count. Having "PIC-responsible important flight duties" is neither necessary nor sufficient for the determination of flight time.
And the whole "logging while taxiing and runup" interpretation brings up more discussion and its own set of problems. Say two pilots take off in two airplanes for the same flight to somewhere 1.0 hours away. They are both in the air for exactly 1.0. Pilot 1 taxies normally and does a normal runup, and logs 1.2 for the flight. Pilot 2 sits in the runup area for a long time for whatever reason and ends up logging 1.5 for the flight. Same trip, same time airborne, but one pilot logs 1.2 of "flight time" and the other logs 1.5 of "flight time". Legal or not, and in accordance with the regulations or not, how does this make sense? To me, it doesn't.
BUT, I also don't get worked up over it. It is what it is. I log my time like most people here, which includes taxi time. Heck, even at work (and many here know where I work), we log "block in/out" time as flight time. It's pervasive and commonplace and doesn't really make much difference. But that doesn't mean it makes any sense.