Final answer:
The substance that should never be used in a draft dispense system for beer is Oxygen (O₂) because it causes oxidation which can negatively impact the beer's flavor and quality.
Step-by-step explanation:
The substance that should never be used instead of CO₂ or CO₂-N₂ mix in a draft dispense system to maintain the quality of beer is Oxygen (O₂). Oxygen can lead to oxidation of the beer, resulting in off-flavors and a decrease in its overall quality and freshness. Nitrogen (N₂) is sometimes used in beer dispensing systems, particularly for stouts or other beers where a creamier texture is desired, and it does not affect the flavor negatively. Argon (Ar) and Helium (He) are both inert gases and would not oxidize the beer;
however, helium in particular is not suitable for this use due to its unique properties such as low density and the fact that it can affect the overall quality of the beverage in unintended ways.