Let's do an experiment.
Say, just for our purposes, WHH is the rule. There is one timeline, and individuals traveling within it to a time where they didn't originate doesn't change it - can't change it, in fact, because whatever they did when they were there had happened or will always happen.
Now let's say: at some point in the past knowledge of the future appeared. Could have been in written form or come from time-travelers running their mouths. Could have been something else. The reason doesn't matter.
The question is: what should one do with the knowledge?
First it must be understood: if the events related are told in their entirety, no one could really do anything about the knowledge other than follow it to the letter, whether one wants to or not. So it's important to note the knowledge must consist of bits and pieces from the future.
In this case, if the knowledge is of a time too far in the future, I don't know anyone would do anything but guard it. If it was of events just around the temporal corner, and if it was negative enough, I'm sure people would immediately try to change things. This would have the unfortunate effect of proving the rule: whatever happened, happened. Most of the time I'd venture to say using knowledge of future events to change them would end up causing those events.
Okay. So now let's limit all the time-travel and future-knowledge to one place on the planet: a small, sparsely-populated island. And, just for fun, we should allow for multiple - and possibly overlapping - past/future exchange events. What can we say about it? Well, that depends how far back people traveled; what they said or wrote; who they met; whether they only visited (and for how long), or stayed permanently; and whether they ever had anything to do with any future events of importance. The act of traveling in time might make one automatically important, even if the events related seem not to be. If they carry no far-reaching consequences, though, they may not make much difference beyond our little island.
Which means our next step would be to make the island important - turning the island into the Island. So we'll say the reason all these time-exchange events happen on the Island and only on the Island is because the Island has something to do with keeping the timeline in order. This has the automatic effect of making a time-exchange event even more important, and the time-travelers, too.
So, I told you all that to ask this: if you knew knowing the past or future would not allow you to change either, what would you do with the understanding said knowledge might allow you to cause these events?
Imagine you're Eloise Hawking. You've just helped some outsiders detonate a hydrogen bomb with the aim of resetting the death of your son - an event you caused in the recent past - and it didn't work. In your hands you hold his journal, filled with all sorts of knowledge, some of which has to do with the future. What do you do?
Perhaps you're Widmore. If, in the future, you find yourself off the Island, desiring nothing more than to return; and you know certain people will end up on the Island - you know the day and date - wouldn't you simply be able to put them on a ship, point it into the open ocean, and wait? Could that have been why Widmore sent those he did, those Naomi had said weren't the right people for the job? When Ben said "He changed the rules" could he have meant the Others have an agreement not to do this sort of thing, not to use their knowledge for personal gain? Could it have taken Alex's death to make Ben realize how Widmore "found" the Island?
I offer the following observation: Jacob and MiB have an interesting conversation on the beach in "The Incident" wherein they both mention something about an end to things. This could imply they know something about this end. Could their "rivalry" have to do with two different viewpoints about what to do with knowledge of the future? They both might know the Black Rock will come to the Island. What if Jacob bringing the ship to the Island isn't a sort of godlike power, but akin to my Widmore scenario above? Perhaps there are people on board the Black Rock Jacob and MiB know will end up on the Island. It's possible Jacob left the Island with a plan to bring these people and their infamous ship to the Island simply by making sure the people who were going to make it to the Island were on said ship.
It brings Jacob's constant mention of "free will" into a new context. He tells Hurley it's his choice whether he goes back to the Island; but if Jacob knows Hurley already goes back, he's lying. Hurley may feel like he's making a free decision, but he's being subtly manipulated by a person who already knows the result. The really interesting point is: it doesn't matter what Jacob does. Hurley is always going to go back to the Island, and Jacob knows that. But the temptation to cause all these events which are important to a future you desire have got to be huge.
This makes me think MiB has the same knowledge, but chooses not to cause these events. He knows the people who come to the Island already, and he knows all their stories. He can't stop them from coming to the Island and fighting, corrupting, destroying; but neither does he choose to cause their arrival, since he so resents the fact they come at all. The longer he lives, though, the more he realizes it has always been Jacob causing these events, and so his "hands-off" approach ceases to satisfy him. Eventually he wants to stop Jacob from continuing to cause these horrible things they both already know about.
Hence, the Loophole: he has to stop a person from causing events he already knows have happened. As long as he can do that without stopping the events themselves, he's free and clear. But he knows Jacob is an important person in Island history, so he has to be careful.
I don't think this is what's going on necessarily; but I love to speculate! And I love to hear what you think of the speculation even more.
(An interesting side-note: there are many things in the much-hated episode "Exposé" which I think are huge clues. One of them might be Nikki and Paulo are lovers who end up in a rivalry. Another could be the Monster can cause people to appear dead, even though they're alive. But I think one of the more important might be the enemy in the "episode-within-an-episode" turns out to be someone whom the "audience" believed to be a major "good guy". Might be a reason to believe Jacob is the really bad one. Just sayin'. I know most people hate that episode, but I think we should be open-minded enough to realize it might be more important than we'd like.)