The theological notion of free will is used in two ways, I think, one in a good way, and the other in a bad way. The good way it's used is: it imputes free agency to people, it imputes an understanding of morality and a moral responsibility for our actions. In other words, God does not foreordain the actions of people; how people choose to act is up to them. This absolves God from ultimate responsibility for human behavior. However, this set-up is complicated by the notion of Original Sin.
For if we are free in our actions then we are free in our actions, but if Original Sin is true than this "stacks the deck" against us as moral agents. We do not inherit this sin nature by virtue of choice, and yet we are to be held accountable for our actions, which are heavily influenced by this innate tendency for behaving badly. A rational person must see the problem with this.
How can a person be punished for his actions if his actions were influenced from the start by an innate predilection for naughty behavior? It is obviously not just. After all, God does not have this innate tendency for naughty behavior, so it is no wonder he is never inclined to do something naughty. So why would a perfect being such as God judge a far lesser being harshly because said being acted in accordance with this sinning nature, the presence of which is clearly not the lesser being's fault? It does not wash.
So the good way in which this term free will is used in a theological context is that it grants free agency and moral responsibility to humans; the bad way it is used in the same context is that it removes culpability from God, which cannot be rational in light of Original Sin, which was brought about by some magical trees God made and a naughty snake God also made. God cannot be judged innocent of the flourishing of human sin, no way, no how (unless we do away with the concept of Original Sin, but even if we do that, we still have problems).
An interesting side-note to this is: God knows this, and his knowledge of this is the reason for Christ's torture and death. Hence, all we need to do is recognize that God apologized for his error by enduring punishment and cruel torture, and he did this through Christ's (His own) atonement on the cross. If we can believe this and believe on Christ we can be saved. Why we can't be saved and on good terms with God without all this razzle-dazzle (to borrow a phrase from kennethamy) and the need for faith, is the real and ultimate theological question.
*
Now, the reason I argue that the denial of free will is dangerous in a secular context (the real world), is because:
Vast groups of people can be controlled only if they allow themselves to be controlled, or if they are controlled by brute force.
Absent brute force, a really useful (and oftentimes awful) philosophical and political ideology must be introduced if one intends to control the masses. History is full of such attempts. Religion is the biggest and most successful non-brute-forceful means of keeping people in order. This isn't to say that certain agents of religion haven't resorted to brute force, of course they have, lots of times; but over-all religious doctrines are devised as a means of encouraging good behavior and discouraging naughtiness without the cudgel; or at least as much as possible without it.
It worked before and it's still working for a good number of people. Note the posters in certain threads who admit that without God's command and God's morality they would do any number of nasty things. We must take such people at their word, if you ask me.
Another ideology (or ideologies) is Statism, or collectivism, or socialism. And in the case of WW2 Japan, extreme militarism. None of these methods of socialization could possibly be set in motion without the consent of the people being governed (and this is not to say that the stick is never used: it is, as is the mere threat of the stick, but it can be easily seen that a bad idea is just as dangerous and has equal if not greater motivating force than a stick or a gun). The best way to get this consent is to convince them that individuality, selfhood, ego, and all those other good things, are actually bad things (or falsehoods).
Convince people today that the really smart folks have figured out that consciousness and selfhood aren't real, that what we think is going on in our own minds is a fiction, an illusion; that we are nothing more than biological, organic machines (some of us "arrogantly-programmed to boot), and you are well on your way to installing your tyrant of choice. It can happen as long as people choose to let the three wise old bald men sitting at the top of the Ivory Tower in Academia decide what is true and what is not.
Every living human being has a right to philosophy, to big ideas, to reasoning and thinking. I'll close with something Frank Zappa said:
"If you want to get laid, go to college. If you want an education, go to a library."
