Useful for what, distributable to who, and how much do they cost?
I like the 50 pound blocks because they are so cheap and easy to store, once you have your normal, survival salt stored, then you can add one or two of the 50s as an almost free, deep storage bonus.
Useful in that the portion is such that it is distributable.
Distributable into caches, rather than having one's entire supply in one block in one place. "Distributed" in smaller, separately packaged (waterproof) pieces reduces the chance of spoiling the whole lot, even if one is relying on a fortress mentality (which is a mistake IMHO).
As an example, your shelves are compromised and fall down. a portion of your kerosene supply ruptures in the accident, and spills upon your salt store.
One or two range blocks open to the kerosene damage would spoil the whole lot - But the equivalent in 3lb blocks, individually packaged in a water-tight fashion... You might lose some (torn packaging), but it is very unlikely that you would lose them all.
and finally, smaller portions can be used as barter. Now, I don't deny that one can do the same thing by chopping up a range block into smaller portions, but the difference in cost is minimal between the two form factors.
I don't know what they cost off hand, but it is negligible - Their purpose is exactly the same: A livestock salt block, available at any feed store... The smaller package is for the purpose of putting it into individual feed trays/mangers (think stalls in barns), rather than a shared range block (what you have).