No jungler should ever be without smite, it's that simple.
Nope. Only downside of not having it, is not being able to take golem, until you have razor.
But after that, its pretty much useless. Exhaust stays great all game long and lets you annihilate enemy junglers.
@nilocy:
Yeah, but it's not a must have for junglers.
When you don't want to have your buffs stolen, this can be achieved with map awareness and wards as well.
The sheer degree of BAD here is staggering.
In rough order of importance over the course of the game:
1) You need the jungling speed. In a the regular fast-non-Olaf-jungler run of Golem-Wolves-Wraiths-Lizard-Minigolems, you'll have Smite at Golem (safely securing it against level 1 ganks, allowing you to progress much faster to other camps) and at Lizard (allowing you to cut short the battle with it to move on and begin your gank). On the Godmode Speed junglers like Rammus, who will finish Lizard before smite returns, you can use it on a Minigolem to nearly instantly clear the camp. Furthermore, once you've reached later phases of jungling, it allows you to smite your buff camps to get back into the action faster when 3 of them are at mid and your solo is starting to sweat. What's most important here is that it allows a 3-5 minutes gank, depending on your jungler, because of how it improves your clear time.
2) You need the security for Dragon and Baron. At 425+25*level, that's over 800 to dragon and 670 to Baron at level 18. And when the enemy team comes tearing out of the jungle looking for a free Baron/Dragon, that's when you're glad you packed smite and your team can be ready for the teamfight with dragon or Baron safely in your pocket (or floating in a circle around you, as it were).
3) You need the security for your buffs. That means that with wards you can steal through walls from nonjunglers, secure your buffs against the Morde on your team who claims he "needs the cooldowns," and that when you gank an enemy jungler you can walk out with his buff either way. Even with wards, you won't always be in position to intercept the enemy doing their buffs or stealing yours. Suppose the enemy jungler has Smite and you don't. You're doing a camp and you see him coming. What do you do? You can't STOP him. Drag it into the bush? He probably has it warded, that's why he's here to steal in the first place. Ditch it? Good luck stealing it off him on your way out when he has smite if you don't.
tl;dr: Smite gives you security and speed, which directly translates into harder, faster ganks as well as level advantage and actually getting your buffs when the enemy jungler comes to play. If you
don't bring Smite when jungling, then you'd best not waste the jungle if there's ANYONE else on your team better than you who wants it. Also, if they jungle with smite they are a better jungler than you. Period. There is no champion I can think of whom you could convince me doesn't need smite; only Shaco and Fiddlesticks don't have their times notably impacted, and with Smite a Shaco will be the enemy jungler's worst nightmare as he steals every buff camp forever with deceive-backstab-smite-dropaboxtocoverescape, while a Fiddlesticks needs it to secure his camps more than almost any other jungler I can think of because he has no burst to finish his camps if someone else shows up.
Any questions?