I believe the purpose for their directions is to avoid damage to the unit due to water freezing.
From a maintainence standpoint, the easiest thing to do is install it with a pair of unions. Then as a part of the winterizing process, just remove the whole thing. That would also give you an input to winterize the rest of the system (i.e. get a 2nd union with what ever is needed to connect a compressor to the system).
I would think the biggest worry if you didn't remove the unit each year is if your shutoff valve ever devolpes the smallest of leaks, that water would eventually make it to the backflow, freeze (if it's in a freezing location) and damage the unit. Of course this would also make it easier to steal. But because it's a DC, you can bury it in a meter box where theives won't know you have a backflow that's easy to steal.