Eh, during the main game I find the smaller potions (and HP healing berries) useful for healing your Pokemon outside of battle. A few of your Pokemon get below half HP & you're a good way into the next route/dungeon and don't want to go back. You're pretty sure your Pokemon will get you through just fine, even if one or two does end up fainting... BUT you have all these extra low healing items and why miss out on potential experience or, if you're playing the first time, caught by a surprise boss battle? Take a few seconds to give your worn out Pokemon some of the healing items you won't be using during a battle thus both using up these filler items and keeping those higher healing items for future battles.
But, yeah, as a pre-order bonus? Sucks.
And honestly it's not that hard to think of a GOOD bonus. SM was close with the 100 Poke Balls, but, how about instead of a100 normal Poke Balls you'll quickly outgrow, why not give us like, 5 each of each type of Special Poke Balls? Fast, Level, Lure, Heavy, Love, Friend, Moon, Net, Nest, Repeat, Timer, Luxury, Dive, Dusk, Heal, Quick, Dream & of course Master. Heck, that's actually only 18 types so you'll be getting less than 100 BUT it would still be WAY better than 100 Poke Balls (heck, better than getting 100 Ultra Balls even).
I can think of plenty of other ideas (a "medicine pack" actually worth a damn, a ton of Exp Candies now that they're a thing, maybe a bunch of TRs and Held items to get an early leg-up in battles, etc).
Though, at the same time, even if they did what I suggest above it's still "meh" in a grand scheme of things as those are just virtual awards of items you can get in-game, some may require more work but it's doable. Like the "best" pre-order bonus is usually the Event Pokemon they have available for download for like the first month the games are out as that's at least something you can transfer up. And while I also don't see much value in steel books, I do agree with
Lemingue that it's at least a physical object that the only original way to get is via pre-ordering so has some "value" even if its just as a souvenir/memento.