As I reread that, I see the potential to get hung up on details, like that a group of all healers won't necessarily do well. That gets into some more advanced maths that don't really enhance the discussion, so just overlook that for now.
The potential to get hung up on details is precisely the pitfall of this kind of discussion. An instanced dungeon queue system really shouldn't be overthought. It's not meant to be end game content, it's just a stepping stone to get experience in groups and a gearing tool. It's your first foray into the wild world of "Group Content". That's how it's used in other games, and likely what Yoshi-P has directed his team to draw ideas and inspiration from. The level of fine-tuning (or lack of any tuning) it sounds like you all are talking about is the kind of thing that happens at the raid level, when players expect a high level of challenge.
5 man dungeons in games like WoW and Rift are just generic enough that it doesn't matter if you prefer to play as a warrior tank or a paladin tank, but specific enough that you can't just walk in with any makeup, hit your "i win" button and collect loot. Go too far in either direction, and you have a problem - either the job elitism ala FFXI, or too much class overlap that there's no point to having "different" jobs.