Optional Fire/Assault Results

[Sir John Cope]

In Mr Grumpy's pinned article on optional rules, the comment for both Optional Fire and Assault Results is a laconic "Do not use unless set by default, for more info see the OR help section."

By OR help section, I assume that the main program guide is meant, and that simply says the effect of both is to average two results, thereby limiting variability.  So why not use these two options?  What am I missing?