Yeah, I'm aware that the jack is for headphones, I just can't recall ever seeing a headphone amplifier with only one XLR output jack. Every pair of balanced headphones I've seen had two XLRs. I have a pair of AKG K702s that are not balanced, so it's neither here nor there to me, for now. I just found it odd seeing a lone XLR jack like that...
I can't see a real need for a balance control, personally. This definitely wouldn't be a feature on my headphone amp wish list. Balance control for a loudspeaker set-up makes more sense, given that speakers are often set up in asymmetrical rooms and such.