In the Moldvay equipment list, it states that "Each cleric must have a holy symbol." Referring to the item, not the design concept. This is interesting because it's sort of a class restriction/requirement that isn't listed under the class, but in equipment. In later editions, there are wooden or silver symbols, but I don't recall any rules mechanic being affected by it. it does offer some roleplaying opps for the player. Spending coin on a nicer pendant might show piety, and the shinier bling could be considered "formal wear" at the great temples.
The main use of holy symbols is usually turning undead and warding off vampires. The former is really just a common house rule, but the latter is spelled out in the Expert monsters section. It doesn't turn the vampire, but keeps him away.
What I find interesting is the comment "they may move to attack the person holding the symbol from another direction." This implies that the "strongly presented" symbol is pointed in a particular facing. That could lead to some dicey tactics, especially since there can be 1d6 of them in a lair!
Continuing the vampire countermeasures, we come to garlic. It isn't in Moldvay, having been added in Cook (haha! "Cooking"). Oddly, there is no description. It must have gone the way of "Detect Invisible."
At 5gp, it seems pretty pricey. I cook a good deal and like to buy fresh ingredients. I probably spend more than I should at farmer's markets and the like, but 5gp for garlic is kinda crazy. I assume we're talking a string of bulbs here, not just the one. Or maybe those wascally merchants know that adventurers are idiots about mundane gear and will pay ludicrous markups for such things! In either case, vampires hate it.
Garlic is the only thing I know of that causes any undead to save vs. poison. If the vampire fails it cannot attack that round! No attack. That's huge! Of course, that implies that the garlic must be re-introduced somehow the following round. I suppose the original idea was the vampire needing to overcome his revulsion a the string of garlic at the door or window or around the maiden's neck to get on with the bloodsucking business at hand. In a melee, I don't imagine a PC simply chucking cloves at the creature.
Wolfsbane is a real thing and it is actually quite poisonous if ingested or gets into the bloodstream. I think some of the anti-werewolf legends stem from the fact that it has been used to make arrow poisons that can drop a wolf or other large mammals in real life! It seems less about magic and more about IT'S FREAKING POISONOUS!
That said, it's important to note that wolfsbane does not kill weres in BX, it drives them off. Still, if I can smack a werewolf on the nose with a purple bouquet and have it flee, I'm good with that! At 10gp a pop, they are not cheap flowers! I would expect that they should lose potency in-game after a time as the blooms wilt and fade.
Frankly, after watching those monster movies and re-reading these entries, I think the next time I have a PC in a Ravenloft-ish situation, I'm going to stock up at the dungeoneering produce aisle!