Well actually, I'd say that the only period in time in which Dragonite ever arguably had no notable niche over Salamence was between the release of Pokémon Platinum and the release of Pokémon HeartGold and SoulSilver in Generation IV (with one's viewpoint on that depending on how highly one regards the move Superpower).
As for this generation, if Dragon Dance actually turns out to be a viable strategy (something I'm unsure of, as I mentioned in my previous post, as well as because the Generation V battling environment hasn't really become stable yet), I'd say that Dragonite would definitely be the better option for that, as regardless of how much faster than Dragonite Salamence may be, it still cannot avoid being revenge-killed by Ice Shard users and Choice Scarf Garchomp, while preserving Dragonite's Multi-Scale isn't that difficult if Dragonite is holding Leftovers, and is placed in a team with a Pokémon with the Magic Mirror ability (and if Dragonite's Multi-Scale somehow gets nullified, then it isn't really in any worse a situation than Salamence normally would be anyway, as in that case they would both get revenge-killed by the exact same Pokémon in that situation), which I believe to be essential on any team focused on a Pokémon which relies on Multi-Scale or Sturdy anyway.
Salamence's greatest niche in this generation would come from mixed attacking with Life Orb in my opinion, due to its Attack being significantly higher than that of Sazandorah, while its Special Attack is noticeably higher than that of Dragonite. Dragonite could still compete with it in this role though, due to the fact that it can also serve a double-role as a lead or revenge-killer against weakened Pokémon at the same time, and also once again due to its Multi-Scale ability. However, I believe that Salamence's Speed and Special Attack advantage would help it more at mixed attacking.
As for this generation, if Dragon Dance actually turns out to be a viable strategy (something I'm unsure of, as I mentioned in my previous post, as well as because the Generation V battling environment hasn't really become stable yet), I'd say that Dragonite would definitely be the better option for that, as regardless of how much faster than Dragonite Salamence may be, it still cannot avoid being revenge-killed by Ice Shard users and Choice Scarf Garchomp, while preserving Dragonite's Multi-Scale isn't that difficult if Dragonite is holding Leftovers, and is placed in a team with a Pokémon with the Magic Mirror ability (and if Dragonite's Multi-Scale somehow gets nullified, then it isn't really in any worse a situation than Salamence normally would be anyway, as in that case they would both get revenge-killed by the exact same Pokémon in that situation), which I believe to be essential on any team focused on a Pokémon which relies on Multi-Scale or Sturdy anyway.
Salamence's greatest niche in this generation would come from mixed attacking with Life Orb in my opinion, due to its Attack being significantly higher than that of Sazandorah, while its Special Attack is noticeably higher than that of Dragonite. Dragonite could still compete with it in this role though, due to the fact that it can also serve a double-role as a lead or revenge-killer against weakened Pokémon at the same time, and also once again due to its Multi-Scale ability. However, I believe that Salamence's Speed and Special Attack advantage would help it more at mixed attacking.