For me, it's important to be able to reach a valve knob(s) to turn on a valve (if it's inadvertently left off or turned off), and to turn off the valve (in the case of a blown hose or a reg free-flow, even during a rec dive, even when using a Y-valve). This was something that was stressed in my open water course, and something that I still consider important (for me) all these years later.
NOTE: An old-school pillar valve helps me with this when I am diving my double-hose reg on a single cylinder (since I wear my cylinder farther down my back when I dive my DH reg, which makes reaching a K-valve challenging).
rx7diver