If this were a solo event, you would probably be right with that. But with an active union, and with only 3 available rooms in experts at any given time, I find myself pretty much never alone even if I tried. And most of my union teammates have far better teams with better debuffs than me as well, so I can just rely on my union mates to carry me and cover all the debuffs. ^_^
So really, that ex slot pretty much becomes useless considering whatever I can fill in that slot is usually always covered anyway.