I never said they have the same role, just that when someone needs a dragon, they tend to use Dragonite. When two Pokemon have the exact same typing, they certainly ARENT completely different; comparisons are almost inevitable. Swampert and Gastrodon have different roles and they were compared all the time in early Gen V. And you're right, many people do use them on the same team, but those are usually on Dragon-based teams, which aren't prominent enought to bring Salamence into top 10 usage.Don't compare dragonite and salamence, they are completely different pokemon.
Dragonite plays mostly as a wall-breaker or rain abuser, while mence acts as a late game cleaner come revenge killer or appears on sun teams to defeat enemy dragons and heatran. While the type overlap is an issue, there is no reason why these two can not be used on the same team, in fact many players do.