I agree that a complex contemporary society needs a complex system of law, and I’m the last one to adopt a libertarian philosophy in which the state is evil (necessary evil or not) and should be kept as small as possible. I accept that the state has a role to play in poverty reduction and redistribution, for example. Laisser-faire leads to injustice.
However, the more rules there are, the more restrictions on individuals’ freedom to act. The rule of law, as opposed to a simple system of legislation, was designed precisely to limit the realm of state action and to open up a realm of society, distinct from the state, in which freedom can rule and laws do not apply. The more laws, the smaller this space of freedom (although one very general and vague law can also reduce this space to nothing). A big state is an enemy of freedom.
The rule of law limits the state and opens up the realm of society in the following way. It limits the number and scope of laws because it allows only laws that are discussed, voted and published according to formalized procedures, and that stay into force until the same procedures result in another conclusion. Moreover, laws in a system of rule of law must respect the fundamental laws, the constitution, and cannot go beyond what is allowed by the constitution (for example civil rights, in a democratic constitution). So the rule of law creates a legal system in which laws are limited and stable. The rule of law therefore creates freedom (from the law) and is incompatible with an ever expanding system of law.
By comparison, the legal system in an autocratic rule by a dictator (as opposed to the rule of law) can result in whatever law the dictator decides, and whatever change in the law he decides (if he bothers at all to use laws for the purpose of his rule). Such a system is inherently unstable, unpredictable, unlimited and expanding.
Another reason why laws should not be too numerous, distinct from the concern for freedom, is that an extensive system of law makes it very difficult to respect the law and without respect for the law, there is no rule of law. People should be given the opportunity to plan their lives in such a way that they can respect the law and can avoid running foul of the law. That’s very difficult when there are too many laws.
Knowing what things the law penalizes and knowing that these are within their power to do or not to do, citizens can draw up their plans accordingly. One who complies with the announced rules need never fear an infringement of his liberty. Unless citizens are able to know what the law is and are given a fair opportunity to take its directives into account, penal sanctions should not apply to them. John Rawls
For the same reason, i.e. giving people the possibility to respect the law, it is also unacceptable to have secret laws, retroactive laws (laws that punish acts that have occurred before the law came into force) and unstable laws (laws that change all of the time). Bad law as well is unacceptable, again for the same reason (by bad law I understand complex, incomprehensible and contradictory law, which are types of law that make it impossible for citizens to respect the law).