While other summoners may be stronger. Smite has a 75s cooldown and up to 2 charges. You can start a fight with it and have one to use in 15s.
Yeah, but even with both uses, it still gives less power than a different summoner, and the other summoner spells give me the power all at once instead of forcing me to wait 15 seconds.
If you're in an unfavorable lane, smite will help you farm. Especially that cannon minion.
If I'm in an unfavorable lane, I'd rather take tp so I can use my health and skills to get entire waves of CS, then back and tp back into lane, or just tp gank another lane to get a snowball rolling.
Facing Hiemer? Smite
Please, please do this if you are facing me when I'm Heimer. I love it. It just makes my day. I have never had an easier lane to bully than one where my enemy devoted an entire summoner spell to wiping out one turret every once in a while. Those games are always so much fun.
Jungle items are cheep for their stats. if you're in a lane and expecting it to go poorly for you. These items can help you back into the game.
The problem with this is that if you are expecting a lane to go poorly, and you take smite, you essentially tell the other guy that he can just force you completely away from the creeps because you don't have a combat summoner to use in a fight with him, and you don't have tp to get back into lane if he bullies you. Your smite is almost completely useless until you buy an item that gives you no stats, so you end up weaker until you finish the full jungle item. I don't want to be weak for that long in a lane I'm already going to have trouble in, particularly when the full jungle item isn't really much more cost efficient than something else I could have had.
Smite does enable you to take camps, but early on your jungler will need those camps (counterjungling is not worth the spell, particularly for the time out of your lane and the risk of the other mid laner coming to stop you), and later on you don't need smite to clear camps.
The only advantage I would give smite is increased objective control, and TP gives that as well while also being a much more useful spell in most situations. It's just a bad pick in the vast majority of situations. You can probably find some situations worth using it in, but they are very rare.