It may have no point in European countries, but I wish we had this law in my country. In some regions where women are under-educated, they get married at a very young age "religiously", without registering by law. These women spend their lifetimes for their families, cannot get away from their husbands so easily because of family ties and customs, but can be dumped so easily. So I think even if you are not married, you should be protected by law, that if you lived a life together and earned together, it must be shared. one must not be disadvantaged so severely, just because she didn't sign some papers.
This is the debate that was had on another thread, following the C4 documentary about Nikah only marriages in the UK, where a survey showed a staggering proportion of married Muslim women in the UK had nikah only marriages (without getting a separate civil ceremony to get legally married under UK law) but were unaware that their marriage was not legally recognised.
The programme showed that confusion arose in part because in other countries a Nikah marriage IS a legal marriage, so if a couple marries in a Nikah ceremony in Pakistan, then moves to the UK, they are considered legally married (because nikah marriage is a legal marriage in the country where the couple were married). However if a couple has a nikah only marriage in England and Wales (I can't remember the exact legal framework in Scotland) this is not a legal marriage and they must have a civil ceremony as well.
To get legally married you are required to give notice of intention to marry with the civil authority, and have the ceremony conducted by a licensed officiant, in a venue licenced for civil marriage. This can be a religious officiant in a religious place of worship, if they are licensed approximately. Or it can be a purely civil affair where no religion at all is permitted.
Legal partnerships should be entered into mindfully and with the consent of both parties - it should not happen by default. By which I mean a legal partnership status should not be afforded by default as a result of long term cohabitation, whether the parties wish to be legally partnered or not. Equally, religious ceremonies should only confer legal partnership status if the appropriate measures are followed (ie notice of intention to marry, licensed officiant, recorded in a civil register).
The fact that couples are in situations where they believe they have legal rights which in fact they do not is very sad, but changing the law so as to make legal partnership something you opt out of, rather than opt into, isn't the answer IMO