Some games have terrible potions to buy. Just look at the Final Fantasy series: All the potions you can purchase are only useful early on in the game, forcing you to rely on magic. While you do find X-Potions and Elixirs that heal you to full on your journey, those are limited in quantity.
The potions are still somewhat useful outside of battle, where they don't require a turn to chug... or at least they
would be useful if it weren't so much more efficient to just
Curaga the entire party and then use an MP-restoring item! Haha
I kind of like the Cooking system in
Tales games (even though it tends to be on the underpowered side), which lets you cook a food item for the party once after each battle, and you can't carry more than like 15 each of the three standard healing items with you at a time. Several games also have "Food" items which are cheap and plentiful which can only be used outside of battle. In a couple of the games that I'm developing, I've decided to split the difference, using similar Food items but limiting them by the party's Appetite (which replenishes while battling).
Items find a similar out-of-battle utility in games where you can't use Healing spells outside of battle. If your healing is relatively cheap and powerful, I find limiting it only to battle is a good design choice. With that being said, though, a lot of players will string out battles in a boring fashion before finishing off the last enemy in order to completely heal their party, so you have to be careful to find ways to limit this fun-sapping abuse of the system.
I love Espon's "expensive, powerful healing items" combined with full heals between battles, at least in theory. But then you run into the issue where the outcome of a random battle can become too definite (and thus uninteresting) if a fully-healed party has no chance of actually losing the battle. There's no risk - it starts to feel like a grind. If the battles are high-variance like Persona 3's, on the other hand, it becomes a great system and it feels incredibly rewarding.
So I think the effectiveness of any kind of mechanic you add when it comes to random battles, loot, healing, etc., is going to be very situational on the other mechanics and dynamics that are being added to the system. The same mechanic can be a gem if it's part of a well-designed, purposeful and coherent set of decisions, and a wreck if it's part of an arbitrary or dissonant set of decisions.