Jacob accepts the job as protector of the source. Now, it is clear that the source is pretty hard to find. Even though he knows it exists, MiB scoured the island and couldn't find it. It seems to me that the source is probably very well hidden, and doesn't actually need a lot of protection.
However, Jacob takes this job seriously. Let's examine how he goes about "protecting the source":
- He throws his brother into the source and turns him into the smoke monster.
- He invites loads of people to the island (ostensibly on the basis of some obscure philosphical point-scoring with his brother), who are then recruited by his brother to go digging to find the source.
- He allows US military to bring a huge badass WMD to the island, (but admittedly does have the military all killed off), but then leaves the bomb lying around for the casing to rust away and the core to leak all over the island.
- He allows Dharma to come to the island, so that they can dig, prod, probe and otherwise poke around into the source with a serious piece of drilling kit, release huge amounts of EM energy, and duplicate rabbits like there is no tomorrow.
- He arranges for a plane to crash into the island, raining metal and burning debris onto the island. Some of his surviving "candidates", think it is a good idea to use dynamite to blow up the hatch built over one of the pockets of "source material", play silly beggars with the button, and eventually cause the pocket of EM material to implode.
- He sends the aforesaid losties on a time travelling trip, ending up in 1977, and allows them to detonate the aforesaid WMD over a pocket of the 'source'
- Finally, he gets himself killed, in another bout of "philosphical point-scoring"
Now a load of people have said that Jacob has a plan, including me, but this catalogue of incompetence does not seem to me like someone who has taken the job of island protector seriously. He appears to be obsessed with winning some pointless argument, and this is pursued at the cost of executing the only job he was ever given.