Assalaamu alaikum,
(smile) In Islam, marriage is not meant to be complicated or difficult, nor is it a sacrament to be bestowed by a religious authority, or a privilege accorded by some state authority (as is seen in Burma, for instance). It is supposed to be a contract between a man and a woman establishing a bond between them that entails certain rights and obligations between the two. It is also supposed to be socially recognized, which is why a walima (wedding party) is important, and why secret weddings are not supposed to occur.
Having a marriage socially recognized means that the rights and obligations of the spouses are clearly established, the spouses are known to be living morally, and the children that might result from the union are protected. Marriage serves not only personal ends, but also societal ones. Marriage is the foundation of strong families, which in turn, are the foundation for a strong and healthy society.
(sigh) I used to believe that a witnessed Nikkah (marriage contract) with mehr was all that was needed. But in our societies today, this is not working. Some men are using the Nikkah to gain sexual access to women, but they are then refusing to acknowledge their responsibilities unless forced to by local legislation. And this is true in places as diverse as Canada, Indonesia and Pakistan. Women who don't have official papers recognizing their marriages, too often end up being shamefully mistreated. The rights of the children vary upon the place. I believe in Indonesia, they have no rights (Ardianto can correct me). However, in my home province, all children are protected by legislation, regardless of whether the marriage is legally recognized or not, or indeed regardless of whether the parents had any kind of marriage or not.
Because of this mistreatment of women, more and more Muslim community leaders are emphasizing the importance of registering the marriage. This provides the women (and possibly children) with some sort of protections. Of course, if God was truly believed in and respected by all parties, the Nikkah would suffice for a woman and her children to access their rights. But as things stand right now, this is not the case.
As far as polygynous marriages are concerned, this is an area of much abuse. And legislation banning them just exacerbates the problem. If anyone truly cares about Muslim women and children, they would be making it possible to register these sorts of marriages.
There are those who would argue that a Muslim woman should just not chose to engage in a polygynous marriage. But if there is a lack of men to marry (and this is an issue in Canada), what are women supposed to do? It is unreasonable to expect women to remain celibate, childless or alone. Nor is this state of affairs Islamic (we are all encouraged to try to marry, and Muslim men are not supposed to leave women who want to marry in this state). What are the options? Engage in extra-marital activity, marry a non-Muslim, try finding a Muslim man in another country to bring to her country, say, Canada (which is incredibly risky, and usually ends badly for the woman and her children) or engage in a polygynous marriage (if you can find a man willing to do this. Most Muslim men don't want multiple wives, you know).
The idea that Muslim men are using their families as ways to make money is morally repugnant. Islamically-speaking, it is the man's responsibility to provide for his wife and children. I have no problem with families who receive some sort of universally-available government support as needed, but the idea that some men may marry multiple wives so that they can take money from them and their children, is not at all Islamic.
Gurufabbes, you asked whether Muslims must have state recognition of their marriages. Muslims are supposed to strive for justice and social harmony. In the context of a country that does not recognize an Islamic marriage as binding on the spouses, it seems to me therefore, that either the marriage should be registered (if this is possible), or if it is not possible, that the couple sincerely seek to find ways to have their rights and obligations made binding before consummating a marriage. In my home province, it is possible for non-married couples to draw up binding agreements (though I don't know if this has been tested in the context of a man who is already married). I don't know about legislation in your country.
If we all, Muslims and non-Muslims, truly want to protect the vulnerable and build strong and caring societies, I believe we can find ways to do so. Personally, I feel that recognizing the Nikkah as a binding and legal document of marriage that would bring with it the societal norms of protection for the vulnerable, would be a useful step towards curbing abuses. Having social benefits paid to the mother of a child (as we do in my home province) would help stop men from taking these benefits. And giving a Muslim woman unstigmatized social recognition, would help empower her to exit from abusive situations.
May God, the Witness and the Truth, Help us to consider how we can best achieve His Injunctions to justice and harmony.