Do they need that option though?You're missing the "it depends" option. This is a question of table culture and it is going to vary from group to group, campaign to campaign, adventure to adventure, and maybe even session to session.
You've never had verteran players pull out the torches and oil the first time their PCs encounter a troll? It happens a lot IME.Generally, I don't really expect a player to "justify" their character's actions; that's not really anyone's business but the player's. And tbh, I've only very rarely encountered a player who didn't make some earnest attempt to keep things in-fiction. In other words, the vast majority of the time this is a non-issue.
SUre that sort of thing happens occasionally, but I've typically avoided this simply by not immediately identifying monsters. I mean, I never say "It's a troll!" unless the party has already encountered trolls or done some research or whatever. The players get a vague description, probably do some checks to identify it, and/or might just charge. Eventually, they say "Oh, trolls!" and proceed with the requisite arson.You've never had verteran players pull out the torches and oil the first time their PCs encounter a troll? It happens a lot IME.
This isn't "using rules to justify roleplay."This poll seems backwards to me: I generally use and see others using in-character justifications for gamey decisions. (For example, writing a backstory to justify a paladin multi-classing as a warlock.)
I do not even understand why someone would need to use a rule to justify roleplaying.
See above: what about situations where it is the player "predicting" things because they happen to know the DM? That's hardly something one can avoid, having narrative preferences is a thing that applies to literally everyone.I answered "always", because I subscribe to the Angry GM's view that metagaming is not a problem, but the symptom of a party-dynamics problem at best. Psychologically, we cannot act (convincingly) as if we don't know a thing we actually know (trained acting being an exception that requires a lot of practice). If you see a troll and try not to metagame, you'll still act in a way that's informed by your knowledge of it.
I think it's always healthier to find a justification for your metagame knowledge. Every Ancient Athenian in the Hellenic period knew the answer to the sphinx's riddles, and they all knew how Theseus used a piece of rope to navigate the Labyrinth. It's not unreasonable for a PC to know the troll's weaknesses, or that they should avert the gaze of the basilisk.
When this becomes a problem is that the GM actually constructed a poor encounter that only works if the "hidden feature" of the monster catches the players unawares, or if the players are taking an adversarial tone to the GM. In these occasions, metagaming is actually a symptom of a larger problem, and the solution isn't forcing players to act as if they don't know what they know, but to resolve the social issues in the group.