Some of the off-the -shelf Simple Syrups are nothing more than HFCS (high-fructose corn syrup) which IS a sweetener, but lacks charming things (like 'real' taste) that actual real sugar contains. Don't be lazy with an Old Fashioned: muddle a sugar cube saturated with the bitters with a dash of water, then add the liquor and garnish. If you're going to use a Simple Syrup, make sure it's from actual sugar, not HFCS!