As was said on several posts you can log anything you want, but personally I would feel like a total fool making an entry in my "record of flying experience" AKA my logbook, for "flight time" that did not involve any flying.
As for logging flight time in the case of long ground holds, well I treat all flights the same. I record the time when the aircraft first starts moving, when it takes off, lands and when I reach my final parking spot. In the long run it all averages out as for every day there is a big delay there will be days where there is hardly any taxing.