The problem is definitely not isolated to newer players. There are max level people leveling alt jobs who can't be bothered to keep their gear up. I've literally been told, "Nah I just want to get to 70 ASAP" after suggesting people put on some real gear, or even to do their job quests. Once I got matched with a BLM in The Vault (57+) who didn't have any of his job abilities after Blizzard III and was wearing iLv100-120 gear. He had several jobs at 70.
So to say as an absolute that this problem doesn't involve max level players is mistaken.
The extent of this problem will of course vary by data center and by the frequency that one queues without bringing friends along (exposing oneself to the possibility of ending up with a problematic party member), however, it is at the very least a problem that exists, and one that could be easily mitigated by just adding a min iLv requirement to each leveling dungeon.