I would not want to convert more than one hardpoint into ordnance tubes...unless I'm going for a gimmick (like 2 Assault Missiles). You will probably still want other things like Single Turbo Lasers or Quad Lasers.
An important thing to keep in mind is what a crippled section of your ship does to the other. For example, on the Raider you can load up the front with missiles, but if the Aft gets crippled, you lose the Target Lock action. That means that the Fore hardpoint probably wants to either not require a Target Lock or be Lasers. I think if you have the Tubes though you either want to spend them on 3 missile options or 2 missiles/1 lasers just to save a bit on the price of things.




And maybe even some of these! 



