Having experienced years of old FFXI (which is still the #1 example of danger present in the open world for an MMORPG), WoW's Hellfire Peninsula (Fel Reaver), and FFXIV 1.0, I think I have a decent grasp as to the pros and cons of "optional" placement of danger. That's right, it's potentially both good AND bad to have such a thing present for any given person to experience. It doesn't have to be one or the other and it's no different than literally anything that is done in life.
To the individual player, it needs to be asked how you view the world within the game. Is the story or chatter amongst the NPCs enough to give you the impression that the "outside" is wrought with danger? That in a more cinematic approach, the Warrior of Light would not be able to kill an entire pack of high level beastmen solo while they're repeatedly stabbing/slashing with their weapons. If how you view the games world as one of danger (along with other things like adventure or excitement), within a lore perspective, then high level and dangerous mobs are a great thing to have. It makes the experience feel grander by giving the world this fact that you must accept as you experience.
Let's say that there was a lv90 aggressive mob somewhere tucked away in a cave, but close to a starter city. No immediate danger to anyone that isn't looking for it. To a new and ignorant player, it instills within your mind the open world makes sense to be seen as dangerous, as the story often points out. It's no wonder these NPCs encounter danger as maybe, since there's a lv90 mob around, there's more scattered about (maybe only within the story).
To an experienced player, the presence of such a mob that is that high in level means that the devs are not above dropping hard to kill mobs on you. Ones that will wreck you. That not everything is designed to be killed by you and only you. Maybe one day, they'll drop some group necessary mobs into the open world (like Hunt ones). It's more of a subtle look into what the devs may have planned or maybe a glimpse into how they want the games world to be.
The cons are obvious. It's unnecessary to do with the current direction that the game is in. Why even bother if they're just going to annoy you and your efforts? I'd argue that a grand majority of players do not care for immersion serving practices, including many who claim they do want it.
Someone brought up the Sylphlands. That is a great example of a glimpse into the annoyance that occurs frequently with high level danger presence. Most of us are already conditioned in understanding that this game does NOT want to annoy us anymore than it needs to. It really would be annoying to take part in something that is otherwise solo, but it happens to cross into lv90 territory randomly. Old school mentality says "it's an MMORPG, expect to occasionally need to find a group for things", but that's not how most look at things today. They could rock the boat on that one with expansion changes easily, but that's about it. We're conditioned to forgive dramatic changes with "big" alterations, like ones found in typical expansions.
Now it stands to reason that they could simply throw in a few high level dangers like said above, without affecting high level questing areas, but then we look at value. It doesn't take much time to just add some generic mob, but there's really no reason to unless they serve some purpose (maybe a quest?). That would just seem like it'd be a pointless overall addition.
If, however, they were going somewhere with that concept. That we'll see more group necessary open world content that isn't just timed like FATEs and Hunts, and more readily available (unlike daily quests), then such additions are welcome for everyone. In most cases, you never want to actually take away from what people recognize as a core reason to play. In XIVs case, you never want to remove the simplicity of having the option to spend time soloing things (like daily quests). You just want to add to them. Sometimes, you could even replace them for something arguably better. Daily solo quests vs repeatable group quests for identical rewards. We had something like that with Diadem, but fell flat for numerous reasons.