It goes on a monopod head and tilts left or right, works like a champ for either orientation. No need for a big L bracket for me.
Most tripod/monopod heads can drop to a 90-degree orientation, with a full tilt feature like the head you link, or a drop-notch on a ballhead. The issue is that when in portrait orientation, the center of mass is not directly over the axis of support, meaning you're fighting gravity which can lead to instability (and with a monopod, fatigue since you're supporting more of the weight). The 'big L bracket' has the advantage of keeping the mass centered.
also if you use a gigapan like i do and want to shoot a panorama in portrait you need the mount on the side
RRS also do an aweseome replacement center plate for the gigapan epic pro if anyone else uses one .
Same deal for a gimbal if you want it in portrait L brackets are handy
I also dont use a normal ball head so dont have the 90 degree slot I use a novoflex magic ball it can go 90 degrees but is more solid stright up especially with heavy lenses