A while ago I posted about J.S. Mill's arguments in favour of liberty, which I generally agree with. I'm not a full-blooded libertarian, however. For example, I think it is entirely appropriate that wearing seatbelts in cars is legally required, despite that being a pretty clear breach of the 'harm principle'. There are two major reasons behind my position here.
Firstly, I - like Mill - consider the value of liberty to derive from its utility, rather than any absolute right. As a general rule, we are best off when given freedom to do as we please (so long as we harm or endanger no-one else). This is not always the case however, and so - in theory - liberty should be suspended whenever doing so would maximise wellbeing. In practice, of course, people's judgements here will often be mistaken, so we should be very wary of giving them such power over us. Nevertheless, there will be times when the evidence is so strong that their intervention could be well justified.
Secondly, mere civil liberty is insufficient to secure personal independence. It would be naive to think that the government is the only (or even the major) threat to our freedom. We are far more influenced by - and dependent upon - other individuals, both economically and psychologically. The question of whether (or to what extent) such dependencies can be relieved or avoided, was one of the driving forces behind Rousseau's political philosophy, and a major focus of the essay I wrote about him earlier this year.
The economic problem is especially obvious. Anyone who truly values individual freedom and independence is committed to the necessity of some degree of wealth redistribution. Without this, less fortunate citizens could find themselves forced to submit to the will of (richer) others in order to satisfy their material needs. Rousseau's answer was that "no citizen should be so opulent as to be able to buy another, and none so poor as to be constrained to sell himself".
The psychological problem is more subtle. Rousseau was particularly concerned by how dependent we are on the opinions of others for our self-esteem. Not only does resting our happiness on such a shaky foundation leave us very vulnerable, but it can also erode our independence as we mould ourselves to please others: peer pressure being a clear example of this phenomenon. Rousseau hoped (perhaps unrealistically) that a well-structured society could help alleviate the worst of this personal dependency, by allowing the state itself to become a source of self-esteem for its citizens. (See my essay for more details.)
I'm not that utopian. I don't think the state can free us from our psychological dependencies, that just isn't realistic (and perhaps not even desirable). However, it does open up another possible justification for state coercion: namely, in those instances where our independence has already been eroded by society/culture.
I think it might be possible to have a law justified in virtue of its effect on societal norms. For example, imagine a society where cyclists generally don't wear helmets (say it's considered too 'uncool' or whatever). Suppose individuals face significant pressure from this societal norm, and so tend to go along with it against their better judgement (just do an intro course on social psychology if you doubt for a moment the plausibility of this). Then I think it would be entirely justified to introduce a law making cycle helmets compulsory (assuming that this has a significant beneficial effect, e.g. it saves lives, etc). In such a case, the introduction of a seemingly coercive law could actually have the effect of increasing an individual's freedom - i.e. by giving them an excuse to break the (previously) prevalent norms. (I say 'previously' because the hope would be that the legislation eventually helps change what behaviour is accepted as normal in the society.)
Such a scenario at least seems possible. Perhaps it isn't very likely, I'm not sure. Social norms probably aren't so easy to shape as that. It would be overly simplistic to think that one could create a perfect culture simply by passing laws. But you might be able to have some positive effect, at least.
What I'm trying to suggest here is that state intervention might at times actually be justified for the sake of freedom, strange though that sounds. But just as people shouldn't be able to forfeit their future freedom by selling themselves into slavery, so (perhaps) they shouldn't be able to take significantly harmful drugs, or needlessly endanger themselves (say by failing to wear a seatbelt). A more obvious example would be compulsory education - though we usually don't treat children as fully free agents anyway.
Paternalism is a slippery slope, though, and one that we should always be careful of. There is no simple rule that will always provide the right answer. Mill's 'harm principle' is a useful heuristic, but there will be times when our evidence is strong enough to justify breaking it on utilitarian grounds. Sometimes government interference might even help individuals attain a greater degree of personal freedom. Sometimes.
Update: See also my posts on The New Freedom and Freedom & Autonomy.