Unfortunately it's very true. I too have had players at 30 or 40 in a dungeon with me confide this will be their "first dungeon as that class". And many more who don't speak but behave in ways more consistent with fighting in a FATE than a dungeon (Trying to rush in an tag mobs first, kiting enemies around the room trying solo them etc).

Personally I try and do each dungeon as I level up on my new class at least once to ensure i have a good overview and develop proper rotations and tactics for dungeon fighting; but I don't exactly represent the norm either since I employ team tactics in FATEs (ie covering the healer and AoE monkey so they can tag mobs/keep the dps alive and making kills/ keep the boss still so dps can flank it etc etc)