I know what you mean. I thought the same thing. The XP-7 is the middle child that everyone forgets about. Portability goes to the SR-71 while non-portability goes to the HR2. There was some poster(s) who found the SR-71 sound to be "scary-close" to the HR2 -- rather than closer to the XP7.
The XP7 does offer great options...home/road use and opamp rolling (I believe.) I went with the SR-71, and I'm glad I did. Here's why. The XP7 is pretty big, about the size of a CD case but much thicker of course. The SR-71 is the size of a cigarette pack. Not a big deal but it's nice. The SR-71 has mini-mini...which is all I'm really using it for -- out of the line-out on my PCDP. RCA inputs seem like they'd be "better," but I don't know...anyone?? But again...I'm running a portable rig. And that's the other point...Headphone listening, to me, IS a portable affair. I don't mean "jogging." I mean, I'm at work, then I'm at home, then I'm listening in bed. A home/stationary rig would be a hassle.
And finally price. I was in no way going to get another $100 by my wife. $300 with the ps.
Even with all my stated reasons, if money wasn't an issue, I may have gone with the XP7. Since I'm not "jogging" with it, but rather just need portability to and from work, the XP7 would have been fine. But the compact size, price, and freedom from extra power cables and wires, the SR-71 is perfect.