Ahhh...ok, that is interesting.If a belt has 6 asteroids it will have no core.
If it has 10 asteroids one of them has a core.
I have a list of Belts that always have a core and every now and then I make a trip round them and collect the cores.
So "does anyone ever visit them ?"
Yes I do
I recently visited a few belts and used the pulse wave but didn't find anything....guess I'll keep looking and make a note of the ones that have cores.
I always felt Asteroid belts were rather underwhelming and wonder if FD would ever expand upon them, maybe increase their size and / or perhaps have interesting / hidden things to find in them. Would make a great location for a pirate base