As long as there is legal recognition for family at all, we need marriage, at least recognized.
And marriage does help to protect people. Think about this. What if someone has two wives, and neither knows about the other? It happens now with marriage being tracked. It would be even more easy for someone to do this if marriage was simply whenever someone said they were "married"/wanted to be recognized as a legal spouses. Who gets what? Who gets to make the decisions when it comes to the death of the person living the two lives, the first spouse, the second, the last, the oldest, the youngest? What if a spouse claimed they weren't a "spouse" at all to avoid having to pay debts of the other because they had access to the accounts and wiped the other out, leaving any debts and even the burial/funeral to blood relatives?
Now, if there is no legal recognition of spouses at all, then you come upon may other problems. Do we make writing up wills and living wills compulsory? Do we force adults to have this paperwork made up for themselves, vice the much simpler "marriage contract"?
I honestly don't see us doing away with legal recognition of marriage anytime in the near future because it really doesn't cost anything to society and in fact, provides plenty of benefits to society, beyond even that small incoming tax bonus to our public coffers due to marriage. Many people want marriage. It is much more efficient and less paperwork (by a lot) for a contract that covers a lot of things that people want that "special person" in their life to have legal say/privilege to.