Imagine the following situation: You're fighting a relatively tough enemy and have lost most of your health. If he lands another hit, you're dead. You've managed to get down his health too, though, and you also need just one hit to finish him.
Now, in a turn-based RPG this would lead to a very tense moment of choosing the attack type with the highest to-hit-chance and lots of praying. In an action-RPG, it will lead to very exciting button-mashing while you try to avoid the enemy blows and land your own deadly strike.
At least, it theoretically would lead to a very exciting and tense moment. In most RPGs, the player will just drink a potion that instantly restores his health and then he'll finish off the battered enemy without any problem whatsoever.
Most games make it much too easy to gulp down dozens of potions in the middle of combat. In Morrowind, for example, you just open your inventory, which pauses the game and gives you time to drink an infinite amount of potions. This means that the combat loses most of its challenge as long as you take enough of those magical insta-heal potions with you. It's just the same in the Fallout games, only there you have stimpacks instead of potions. But drinking lots of potions in the middle of a battle is neither realistic nor challenging.
Some games, though, do it right. The Gothic games and also Risen make it almost impossible to drink a potion in the middle of combat. Well, you can try to drink one, but you have to be far away from the enemy because you're completely vulnerable while you drink, since you have to put your weapons away and watch a drinking animation. When fighting multiple enemies in a narrow dungeon, drinking a potion is usually suicide.
Another great example is Jagged Alliance 2. Yes, there are no magical potions since it's a modern real-world setting, but there are first aid kits. And while those usually restore health in other games, in JA2 they just bandage your wounds so you don't bleed to death. If you get hit, you lose health, and you won't be able to heal this wound until the end of the battle. This makes the whole thing a lot more tactical and interesting. Using cover and avoiding to get hit is essential to winning the battle.
Another problem with the use of potions is that usually, enemies don't use them. Fallout is a nice example for a game where enemies do use stimpacks if they have them in their inventory, but other than that I don't know of many other games that have this feature.
Generally, I'd like to see more games where the use of potions during combat is either restricted (maybe by restricting access to your inventory and only allowing you to use items strapped to your belt) or where there aren't any insta-heal potions at all. It would make most combat encounters a lot more challenging and interesting.