Hull blocks have no purpose ? :o
They're what keeps your ship together. How is that not important?
Anyhow, I wouldn't say that any of the modules are really useless. It mostly depends on the situation – modules can give you great flexibility there.
If shipbuilding and modifications in the field get a bit more limited/costly (which I really hope for), then that flexiblity will become even more useful.
While I also wouldn't normally use cargo-modules, they can certainly be handy if you have to quickly expand your cargo capacity without modifying the ship.
In my eyes the modules have a similar role as energy-redistribution has in some other games.
I do agree though that some special blocks for turrets could be useful. Maybe not to modify the number of turrets but there could also be blocks that modify the turrets placed on them in some way. (like making them bigger)