This is just my take as to when I choose Prismatics (but, this does not apply to the Python). I prefer Prismatics in the slot lower than maximum both for trader and combat builds. The reason why is that it frees up you biggest slot for cargo or shield cell banks. In the combat build, it can give you near full charges and bail ya out of the Oh, Carp moments.
Everyone may not agree, but that's where you will see them in any of my builds; except for the Python. The Python has SOooooi much energy to spare that you can max it without gimping a sound weapons loadout.
Oh, and when Prismatics go down, they take an eternity to come back online; which has made me shy away from them when doing strict combat.