The drawback to disallowing camping in dungeons is that the dungeon design has to be targeted at a specific (average) player skill level. It can't be so large that the average player can't get their party to the end without running out of spells and potions. Or worse, having to backtrack to the entrance to replenish and start back in again.
The teleportation mechanic means a huge dungeon can be designed on its own merit, without having to cater to different player skill levels. Noobs can teleport to camp frequently, more experienced players will push further before camping, or maybe not camp at all.
There can't be intermediate rest points either, at least with the current Act 1 design, because Larians wants us to interact with all companions at the camp site, Not just the ones we've chosen for the party. Also vendors like skeleton dude. It wouldn't make sense for the others to suddenly show up at a designated camp area inside a dungeon.
An intermediate solution is to allow teleportation to camp from anywhere, but you can only return to those runes/fast-travel locations. This would encourage players to press on in the dungeons until the next (appropriately spaced out) rune, but also allow teleportation back to camp anytime if you're willing to do a little bit of back-tracking.
