Salamence should be used more, simple as.
If Stealth Rock is on Dragonite's side of the feild, which, as an assumed battle condition, should always be... well... assumed, Multi-Scale is useless. You get NO BENIFIT.
However, if SR is on Salamence's side of the feild, you still get use from Intimidate.
Mence dosen't NEED to set up a DD to be a threat. Mence has 20 more Base Speed, and a significantly higher Sp.Atk.
Mence dosen't NEED Roost.
Any Dragonite running 3 or more attacks should be a Salamence. Simple as that. Unless one of those attacks is Dragon Tail.
Roost + 3 Attacks is easy to handle, because a lot of the teir outspeeds Dragonite. It might get one KO, but will lose Multi-Scale as it does so, and then be revenged, or forced out, only to take more passive damage and get further from Multi-Scale.
DD + 3 Attacks is horribly outclassed by Salamence. Especially with SR on the feild.
CB is done better by Haxorus and Salamence, due to higher power and speed, respecitivly. Especially as any form of passive damage means CB loses the reason you'd use it in the first place, Multi-Scale. Switching in with SR or Sand up means you have an inferior Dragon.
Unless you're using it for Extremespeed, of course.
All MixNites should be MixMence. Especially with less Blissey to Superpower. The higher Sp.Atk and Speed makes this a no-brainer, as well as the hit and run nature of Mix sets.
The only sets Dragonite does outright better is Roost + DD sets which lack coverage, and Dragon Tail sets. Some freaky StatusNite as well, but... who uses Dragonite for Thunder Wave?
Mence's decline is probobly partially because of Excadrill outspeeding in the Sand and OHKO'ing to revenge kill, Scizor still being popular, and because people see Multi Scale and think Dragonite is better. Which its not.
Dragonite, IMO, suffers the same overhype as Terrakion used to. Remember the Smogcast where they said Terrakion kills everything? They seemed to forget how hard it is to set it up, get it in, and keep it alive, seeing as it's weak to 75% of the metagame's attacks, and all status stops it dead.