I think the main reason behind this discussion, is due to loot ninjas.
You open a chest in a dungeon, its an item for your Specific class, and you desperately need it!
But an obvious Lv50 rolls the greed on a the armor/weapon that they dont even need (its likely going to end up on the Market Boards).

Then there's the Loot Cheaters, who seem to roll 90+ on every single armor/weapon obtained from a chest.

In a Solo dungeon, you wouldn't have to worry about getting nice, unique green rated gear, cause you'd be the only person there.

Story Quests and Class/Job quests are Single Player quests, sure. But the gear you get from them? Its normal, its gear you can buy from NPCs or make through crafting.

Perhaps, if they limited the 'solo' dungeon to once a day, or once per class, and you only got gear for that class, that wouldn't promote too much Solo play.