We need strong courts.
One reason is that legislators not infrequently pass legislation they believe to be unconstitutional, trusting the courts to stop them if they cross the line. For example, both Colorado's John Salazar, and Pennsylvania's Arlen Specter voted for the Military Commissions Act of 2006 despite the fact that both men stated that they believed it contained unconstitutional provisions, and left the courts to sort it out. The same thing happened when the South Dakota legislature passed a law banning abortion (which is now going to voters) which they were well aware was unconstitutional. If courts unduly defer to legislatures, we are left with unconstitutional legislation. Strong, independent courts are a better alternative.
While I believe that this is disgraceful behavior, it does happen. In fact, it is part of a larger phenomena which I call the "sovereignty of the group". Almost any group of decision makers too large to fit around a boardroom table is almost incapable of making decisions that honor external constraints. They vote for what they want, constitutions, laws and rules applicable to the decision be damned.
Fortunately, very few organizations in our society are run by large groups, and large group participation in decision making is carefully channeled most of the time. Private corporations are generally run by CEOs subject to oversight from a modest sized board of directors basically limited to hiring and firing them. When this doesn't happen, as in the case of the fifty member American Red Cross board which actively particpated in many decision making issues for the organization, organizations start to malfunction, as the Red Cross did during Hurricane Katrina. Shareholder or member participation is generally limited to elected a board of directors than in turn elects a small number of officers, and one or two occassional specific issues posed to them by the board or a shareholder group.
All but a handful of the nation's local governments, all very large cities, are run by small groups, rarely more than a dozen city council members, who operate by consensus most of the time and take direction from city managers or mayors.
State legislatures are over the sovereignty of the group threshold. But, the power of a governor to veto legislation, the strong role often given to small groups in setting the agenda of the bodies, and the role of small committees, as well as court oversight, limits this impact.
Citizen initiatives, in which proposals get on the ballot without that kind of check, and where voters on the issue cannot be effectively constrained to act constitutionally or lawfully in their yes or no votes on the legislation, are not surprisingly, frequently found to be unconstitutional in court challenges.
Of course, by far the largest legislative body in the nation in absolute numbers is the U.S. Congress, which is at the upper end of the size of a body even capable of conducting deliberative activity, even strong rules committees and a strong leadership structure. Fortunately, because it is at the top of the system, most of what it does is by definition legal. It even has the power to unilaterally pass laws overriding treaties, and like state governments, can be constrained to some extent by the Presidential veto power. Only when it abridges the U.S. Constitution does it end up in trouble, and independent courts can constrain it on those occassions.
It also helps that the U.S. is a large nation, so it can get away with largely ignoring international constraints. These constraints are greater in most countries, but those countries, with parliamentary systems of government, have much smaller groups making day to day decisions, and hence are less prone to suffering from the sovereignty of the group effect.