I think it would be a lot of work on the technical side to have a 5th person, even in a minor role. And the way SE sees it, 'support' is rolled into existing classes in bits and pieces rather than having a dedicated slot.

What I'd rather see is a potd-like instance for DoH/DoL - where instead of fighting, you craft or gather to solve various puzzles (bribe or put to sleep that monster, fix that machine, etc) to move on. While it might sound like more work at first, I believe it would really be much less - as something like allowing a 5th party member would require not only new actions for 11 classes and redesign of the duty finder system, but also adjustments to 64 dungeons and potentially some story instances and such. New interactions like harvesting from corpses would also very much be non trivial to add, and then you'd have to balance drops and so on.