The secondaries aren't a problem because only one of them can be selected at a time. Those keys can be left to cycle as they do now (I was unclear about that before). The difficulty is with the primaries:
Right now, the player can have a maximum of two primary banks, firing in a total of three combinations:
1,0
0,1
1,1
If we have three primary banks, the number of firing cominations grows to seven (assuming one bank is always active):
1,0,0
1,1,0
0,1,0
0,1,1
0,0,1
1,0,1
1,1,1
That many key presses to get the desired firing combination would be annoying, and contribute greatly to pilots getting slagged while having to pay too much attention to cycling through them all to find the right one. Instead, it was suggested that each of the three banks have a toggle key, so that one could turn on or off any one of them without worrying about the other two. It would work quite well, from the player's perspective.