I know I am a bit slow on the draw but anyway....
Ironically- the X-ring is NOT more expensive than the O-ring. My FIL got quotes from TracMac, end of 2009, and the negligible R30 or R40 difference saw him opting for an X-ring. Overkill it may be, but for the same price (or a negligible difference), rather have it and not need it than need it and not have it. But, you are quite right when saying there is nothing wrong with an O-ring either.
My 2million Zim dollars worth.