Would you F-TM a 77.8% ivs 15A legacy Dragonite SW/DC (3171 CP)?. I have no experience with a DT/DC Dragonite, but its ranked A- while SW/DC is C.
I have two others DB/DC (93.3% 3326 CP) and DB/DP (91.1% 3219 CP) and they are my go-to, all 15A iv.
Wherefore do you need your dragons? As a gym-sweeper, one good dragon (your DB/DC) should be great. No need for another one.
If you want more dragons,
i would concider to C-TM the DB-DP-Dragon - i did this and love my DB-O at 3470 CP.
The SW-DC would be least priority in my list, because of the low IV. If you really want to use it - (maybe there is a solo-raid where you need 3 generalist attackers) - Why not? In this case, i would use a TM.
I've been happy with F-TM'ing the 2 of my 3 'Nites that had SW - but I've been using them to solo Machamp. My only slight regret is if sweeping a gym and there's a (degraded) lapras, fairy or other ice defender, then having the steel move would be faster to not bother swapping out, just use SE steel and charge up the special attack to 1-hit KO the next defender!
Dragon Tail is a ridiculously good quick attack. It increases the damage of every charge move by at least 2 DPS over SW/DB (when calculating charge up time and damage added to the charge attack damage and time). Even Gyarados with DT dominates DT with bite. Against foes that are weak to Dark, it's a tiny edge for Bite, but generally, DT is way better.
DT/DC is kind of an odd ball. It is on paper a very good moveset with stab and DPS, but you are combining a grandma speed quick move with a lightning charge move which doesn't fit many usage scenarios particularly well.
My best dragon (15/15/14) has this DT/DC moveset, but I almost always prefer my DT/OR, DT/H or DB/DC. DT/DC I only ever use when the above 3 faint.
There is a case for DT/DC in gym raids though because the damage rounding would heavily favour DT over DB and DC makes dodging so easy. Then again, it's an odd ball given how DT makes dodging hard!
I think changing SW into a DT/DC Dragonite is always gonna be relevant.
It creates a cool legacy move set that was not possible at the time, as you are combining a old move with a new move, which makes a set that was never obtainable untill TM's.
Sure the IV are not great, but it will still do its job, if you need another dragonite.
Anyway, its not going anywhere.
If you have enough TM's then go for it, theres allot worse options you could use a TM on.
If you dont have many TM's you could easily wait abit till you have more and no other important things on your to do list regarding TM's and moveset changes.
Honestly don't know where all the varied advice is coming from. If you have a comfortable buffer of fast TMs (say 5 or more) as well as having the realistic prospect to get more, then I'd be doing it. If only for the reason that you will kick your ass 100 times over if DT becomes legacy. DT/DC best defensive dragon moveset by far and it's unlikely that CP will be of such importance in any future gym alterations. But expect alterations, as the current system is rubbish.
