The lockout is in place to discourage people from leaving groups early for petty/minor reasons or "fishing" for groups (joining and dropping until they find one they like) since this is pretty inconvenient to the other people in the party who then need to wait for a replacement.

I don't see how you could possibly expect them to implement an exception for "special cases" like the one you describe. Do you expect a GM to manually review your situation and determine that yes, you didn't deserve the 15 minute penalty? I understand that it's inconvenient in situations like yours but I assure you it'd be far more inconvenient to have tanks and healers just joining and leaving random dungeons constantly until they find one with exact party makeup and gear they're hoping for.