I don't seriously mind not having a keel weapon slot - but even more so, if there was a stern weapon slot ( as per prow, but covering the aft half / quarter depending on weapon ) .
But we were thinking more along the lines that the keel slot would be useful for carrying a single landing bay, that would not then leave you with an asymmetrical weapon layout that a single landing bay would frequently give, if taken in a broadside slot. For that matter, it would even make perfect sense that such a "keel" landing bay, is a non-removable item, that way it would not even give the same coverage that a standard keel weapon slot would normally give.
Now think about using a Lunar, as I mentioned, with one of the port broadside slots moved to the dorsal position and one of the starboard weapon slots moved to the keel and filled with a non-removable landing bay ( that doesn't allow for aft coverage that a standard keel weapon would give ). Can you really say doing something like that would make it OP? If so, what advantage would the layout give over any other single landing bay, other than eliminate the asymmetrical weapons issue?
Being able to get a double broadside might make sense with multiple friendly ships are around, where you have allies that can cover or help you, but a lot less desirable in a ship that is less likely to see "fleet" actions and more likely to see single ships ( or at best dual raiders ). As such, it would be a much better choice for a second or even third ship, than a first ship.
For a first ship, having weapon slots that are not limited to firing in one direction like keel, prow, and dorsal ( depending on the weapon ), gives the ability to focus fire that having a double broadside can not give ( without the penalty I list below ), in single ship combat that is more likely to be found when starting out. You see, in single ship combat, with double broadsides, half of your broadsides are useless at any given moment anyway - taking up ship space, SP, and power, that might be better spent, when not in combat. This was probably the biggest deciding factor for our group deciding that the Lunar Class, had a poor weapon layout. Such a penalty is less of an issue, after the RT group has progressed enough to own a ship that has enough power, SP, and ship space to spare - not to mention enough skill to fight a fleet battle.