Actually you are correct, capturing accurate volume at bid and ask would be somewhat inaccurate in very fast markets.
But I wonder , at the end of the day, how much difference it really makes 99.9% of the time.
Anyone use Market Delta with the ib feed (does it even run off that?) and make a comparison?
Lets face it, this is not a game where absolute accuracy means a damn, so unless the data is really skewed , I fail to see what the fuss is about.