The idea is that your ship can specialize, that you must make trade offs to excel in a specific area. For example if you want to be an amazingly tough tank your damage will suffer and if you want to be a speed demon your tank will suffer. Perhaps you prefer to be a generalist in which you can do many things but you are also average, you don't do a lot of damage and you cannot absorb a lot of damage. This philosophy is followed fairly well through all ship types except for one, the mining ships. Why is that the case?

With the recent EHP changes to mining barges they no longer follow this philosophy. Miners are no longer required to fit a tank at the expense of other factors in order to ensure their safety.

but the higher tank of the skiff is irrelevant, so they aren't trading anything, they've just got high yield/cargo and don't sacrifice anything for it.

of course tank is only relevant to suicide ganking.
because tank is irrelevant if you aren't getting ganked. these aren't combat ships.

as for the rest of your post, what are you talking about?

Key word there is just after the changes. Right now exhumer kills are at an all time record low.

... which is the problem that I want to see fixed.