I think, it is enough to manage this finctionality by thunar-volman package: if it is not installed guess volume management turned off), if it is installed guess, that Volume Management is enabled (the configuring Volume Management tool is available from general XFce4 configuration menu). So, the checkbox and link could be removed from Advanced tab of thunar Preferencies without any loose of functionalty.
Additionally: yesterday, I've made a check on a system with configured automount. And find that volume management menu is available from XFce main configuration menu independently on it's status (which now is to be set in thunar). More: doesn't shows status (is volume management enabled or not). Looking an steps for creation of user-defined association DE filemanager with favourite application I think that it will be good to make volume-managet standalone (independent on thunar) applications. My suggestions for thunar device access I'll present in a new bug.
Excuse me, I was mistaken. This checkbox works independently on thunar-volman plugin.