There are also proxy jungle strategies. Riste has been taking smite to lane off-and-on for the past month or so. (He built Cinderhulk and used it to proc Phase Rush.) Xianji is another diamond Garen who proxy jungles and makes a jungle item as part of his build. Notoriously, a player named Max Shmit at one time had the most Garen mastery points, and he always takes smite to lane.
In seasons past it was not uncommon for supports to run smite in order to build Cinderhulk as well.
There was also a top lane Smite + Teleport strat designed to maximize income.
In the current season: a midlane smite strategy designed to take advantage of the increased gold in the scuttlers. Incidentally, smite can serve as a short cd heal spell, even if the scuttlers aren't actually killed.
In the current game, one good reason to take smite, is that it procs the Unflinching rune. A laner can keep up almost consistent %32 Tenacity without mercs, from lvl 1. For this reason alone, Smite can be a legitimate laning choice.
Additionally, canon minions now have increased gold and are considered "mini-objectives". So, smite is "canon minion tactics"; it is difficult to deny canon minions (now over %20 of the farm gold concentration) even to a pure melee fighter, if he brings smite.
The smite spell brings a lot of utility. Even if it just brought one extra minion every 70s, that might be reckoned sufficient reason to run the spell.