The VSI is a most accurate instrument, and is very quick to show any alteration of altitude, just ask any Glider Pilot what they use to detect lift or sink.
I have a VSI and associated capacity flask, that I use for demonstration purposes. If I hold it near the ground, then raise it above my head, it will read +100 fpm, then slowly drop to zero. Lowering it again to floor level shows -100 fpm.
So if it accurately shows a height difference of 6 feet then that is good enough for me. An altimeter on the other hand would take one minute to change 100 ft, and it might not even do that, as they are well known for having some in-built friction.
The AI is also very insensitive, we are talking about fractions of the 3mm dot, to produce the same 100fpm change.
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