Kind of inspired by a conversation on the other thread, I'm the DM that doesn't let people stand in the back never getting attacked. Enemies use tactics that make sense to me. I don't use dungeons very often, so there are frequently no choke points. Enemies will work together so if the tank in front has sentinel, multiple enemies (frequently sending the weakest first) will rush past to get into the best tactical position. Enemies will regularly flank to get to the PCs with the worst AC. Intelligent humanoids will always have ranged attacks even if they are not included in the general stat blocks, all my orcs carry javelins (and I allow PCs and NPCs alike to throw multiple javelins).
If that sharpshooter archer elf always stays in the back, enemies will hear of this tactic and feint an attack from the front with low-level troops while sending their heavy hitters around back to focus fire on the guy who took out one of their champions a couple of weeks ago. Other times intelligent enemies will recognize which PC is the healer and focus fire on them first.
Not all the time of course. But for every tactic a PC will take to be "safe" there will be times the enemy will use tactics to counter that. Casters and archers are in as much danger as the front line fighters. While I don't run a particularly lethal campaign, that caster or archer in the back is just as likely to be targeted as the front line. Perhaps even more if they think being 100 feet or more away from the rest of the party when the fighting starts is a good idea.
If that sharpshooter archer elf always stays in the back, enemies will hear of this tactic and feint an attack from the front with low-level troops while sending their heavy hitters around back to focus fire on the guy who took out one of their champions a couple of weeks ago. Other times intelligent enemies will recognize which PC is the healer and focus fire on them first.
Not all the time of course. But for every tactic a PC will take to be "safe" there will be times the enemy will use tactics to counter that. Casters and archers are in as much danger as the front line fighters. While I don't run a particularly lethal campaign, that caster or archer in the back is just as likely to be targeted as the front line. Perhaps even more if they think being 100 feet or more away from the rest of the party when the fighting starts is a good idea.