I don't think that the Black Rock journal was key to finding The Island or solving mysteries or anything. I think Widmore (who already knew how to find it and knew much more about it than some primitive sailor) wanted it for two reasons: as a relic, and to keep it out of the hands of outsiders.
DHARMA people were closer to MiB (meddlers with The Island's powers, technology, exploiters, Men Of Science) and The Other were closer to Jacob (primitive, low-impact living, taking indirect orders from Jacob, "Templers" (sic), believers, Men Of Faith). Of course, each player had men in the others' camp, but in general, DHARMA was in bed with Smokey. Doing away with all of these meddlers (once any potential candidates were given a chance for redemption) would serve Jacob's cause.
Kelvin (and other aspects of DHARMA) carried on after The Purge probably because they were hidden. Other aspects (like The Flame) were co-opted by The Others. If TPTB don't explain the pallet drop, for me it will be one of the biggest non-critical-to-the-show disappointments for unexplained events.
I feel pretty satisfied with what's been explained so far VS what I am able to piece together/speculate myself VS what is completely unexplained.