This practice hasn't been discussed lately. Long ago, perhaps as long ago as when Brad was still with us, it was. The concept was that having mixed levels of mobs leads to having mixed levels of players in the same areas as a counter to having every area being 'just' for levels X to X+5 (for example). With the result that almost nobody past that level range ever returns to the area. The goal involves fostering interactions between newer players and the more experienced ones. Letting players see how much more powerful they can become, ask questions about the specific area/mobs they are fighting, possibly make new friends. It also gives lower lvls an easy option to ask for help from higher levels who are right there when help is needed.
In those discussions, many players expressed appreciation for having the occasional much-higher level mob wandering around. It keeps one alert and watching out for danger, even when you reach the point where you're still grinding for decent Xp but have little to no concern about dying to the mobs you're fighting.
Admittedly, there can be areas - like you point out - where it isn't appropriate due to having no way to see the danger approaching, or to avoid it if they do.
So pointing out such situations is certainly valid and a help to the Devs. Thanks.
See ya on Terminus
There should be some kind of indication that a high-level monster is in the area, maybe an audio clue or environmental clue. Just slapping an instakill monster in a cave of level 8 monsters without a leadup is ugly. Give that high-level monster a sense of prestige for being there.
Jothany gives a pretty good explanation on WHY they need to be there, and they DO need to be there. We have Zirus in Thronefast and Bolt in AVP to name a few, the suggestion to just remove them is wrong... that's what WoW did and one of the reasons we are here looking for something different and not in Wow. Remove it, make it simple, is not the right way to solve issues.
The FIRST solution is to watch out for him, realize Terminus is a dangerous place and you need to position yourself, your group your pulls ALL of it accordingly. This is not WoW, if you just blast in and start slaughtering you will die (I hope)! Yet as you point out This isn't always possible in the dark
The next, one that's in progress (This is EA remember) look at the tools we have, or will have that can alleviate the problem, 1) Darkness is still being worked on, lots of progress has been made. 2) Player tools to alleviate darkness are still being worked on Magelight looks like they JUST fixed (made some progress on) the massive mana drain it had, used to suck my wizard dry in seconds, now it only sucks 50% in 5 min. I have heard talk of Rangers with flaming arrows and other such player tools. Torches are not the end all for light sources, just a place holder. Pantheon is planned to emphasize PVE (Player Vs. Environment). Darkness is part of the environment, and the darkness NEEDs to be dangerous.
I really love SkarloRDs suggestion of an audio clue, if it's a LARGE monster, would he shake the earth as he walked nearby?