I was playing jungle today and I smite the red buff and I thought to myself, "How good would smite be on an ADC?" Skirmishers saber does true damage when used on a champion so how good would that do against an enemy ADC with barrier or a support with a decent shield. Chilling smite is great for slowing and chasing the enemy. Rangers trailblazer is great for wave clear. The second summoner spell is up to the person but I think smite would be perfect on an ADC.
Smite would probably be really good on an ADC. However, the biggest problems with ADCs are survivability. Everybody wants to dive and kill the ADC and without flash or heal/barrier it becomes really hard to stay alive. Now it's even to the point where the adcs kill themselves with autos because the enemy has gromp buff + thornmail and being dove by everybody. Plus the item costs quite a bit.
Nah it would slow down your build too much in my opinion. I mean you figure at some point its your skirmisher's sabre against the enemy adc's pickaxe plus summoner heal.
It's impractical on ADCs, considering you don't get good stats from it until the >2k upgrade.
So you'd either not bother with upgrading Smite--and being able to use it outside of single minions/monsters--until you have some damage from other items, or you start off with an item that won't grant stats and will hold you back against your opponents.
Plus, the two champion smite effects aren't that useful to Marksmen.
Costs Heal and costs lives
Jungle items in general are not nearly as useful as standard ADC itemization.
Devourer is only good if you're clearing the jungle at a fast pace, and if you're laning you'll get hardly any stacks. BotRK is also just straight better than Devourer if you want an attack speed item for an ADC.
Warrior is inferior to Yoummu's due to the loss in active and an inferior build path. (Again, you aren't clearing jungle camps regularly like a jungler so the machete is a wasted investment).
The smites may be useful, but really you'd have to give up heal for any of them. Skirmishers may be nice for 1v1 situations, but it's not going to save your bacon in a teamfight like Heal does.
I don't think Smite would work all that well on most Adc. A lot of them(if not all of them) need very expensive items as early as possible. Taking the time to build a smite item instead of a 
or
would be counter productive. The small amount of cc or true damage wouldn't be worth it.
I have tried it before. It is far from useless, and I am sure a more skilled player than me could make great use of it. But there is a reason it is rare. Flash + Heal or Flash + Ignite are both very fine choices, and it just feels like you are missing something when you go smite. But by all means try it, perhaps you can make it work. #YOLO
EDIT: One thing I forgot to mention: It does help with gold more than you would think. Never failing to last hit a cannon minion is nice.
Woops. Forgot hit the post button.
I was going to say that on laning ADC's it's a no go. But ADC's in the jungle are not impossible, though slightly impractical for ranked play.
The best ones for the jungle in order of effectiveness in my opinion: 
or 
--> 
--> 
. The reason the first 2 work is that they are less gold dependent than standard ADC's. Urgot has a displacement in his Ult and slows on his EWQ combos as far as CC which isn't bad, and his passive and shield keep him healthy. Quinn and Twitch are good assassins and can use their Ultis to gank lanes very effectively, but Quinn has a much easier time in the jungle thanks to her blind and she can use her W to check for counterganks. Twitch has the advantage of stealth ability, which makes him a pretty potent ganker, but his clear is the scariest as you don't have a way to reduce damage to him besides using your range to soft leash camps. Graves can survive quite well in the jungle thanks to his passive, but he is a very weak ganker so I have him at the bottom.
I haven't tried some of the other ADC's I think would be viable in the jungle. Most of them would struggle and would still have low pressure to lanes.
Some champs that might be worth trying (might): 
(passive with Krugs synergy + Higher base health and armor) or 
(passive + bushes = more damage, Ulti=easiest gank/kill secure ever)
