Seems to me that being strong and active is certainly a prerequisite to being able to work successfully. Their ability to work successfully drives pitch rate, right. Anyhow, I am impressed with the fact that yeast can sit in the fridge for such long periods of time and then fairly easily be brought back into working order. Very cool.
The yeasts ability to work effectively is vitality.
Viability are the cells that are still alive that can be brought back to a vital state.
Pitch rate is the number of vital cells you are putting to work in your wort relative to the volume.
If you revitalize an old sample of let's say 50 billion cells in 1-2 liters of starter wort and you double or even triple (optimistic) your cells numbers you end up with 100-150 billion cells.
If you pitch that into 5-5.5 gallons of 1.065 wort that your desired pitch rate said you needed 300 billion cells you are under pitching your beer by half or more.
Now since what you did pitch is at a high vitality the result of you under pitching may be subtle but include:
Elevated levels of off flavors (maybe or maybe not to the taste threshold)
Reduced levels of the flavors and aromas you were expecting the yeast to contribute to your beer (if your brewing a beer with a relatively neutral yeast flavor/aroma profile or the Malt/Hop are to dominate this may not be of a concern).
Accelerated mutation or drift from what the yeast was designed to perform like (attenuation levels, alcohol tolerance and flavor/aroma characteristics) which should ultimately reduce the number of generations you harvest.
It's obvious the yeast can take a beating and they will make beer even in a less than ideal environment or state but if you plan to harvest multiple generations or you believe pitch rates can help you make a better product then it may be worth consideration.
