One of the most common reasons to combine a hard liquor with Mountain Dew is for an "extreme" tasting drink. Mixing gin, tequila, or rum can make it taste less like soda and more like alcohol. The assertiveness of the hard liquor cuts through the sweetness and syrupy flavor you might experience from drinking only soda combined with vodka or whiskey.
Some people find that digestive bitters help temper the sweetness of sugar-laden beverages such as Mountain Dew while accentuating taste nuances found in different spirits when mixed alongside other flavors (such as infusions). If you are interested in this approach to cut through the sugar, then I recommend using one packet of bitters per 12 ounces (or one bottle) of your chosen beverage before serving.