I'm trying to think of the legal benefits I would have had from marrying the DC's dad and I just can't think of any.
If your relationship stays strong forever and your wills are watertight, there might not be that many (though there is IHT and a couple of other things. This ruling doesn't actually overturn anything, it's just paved the way for the possibility.).
But that's not when you need protection. You need protection when things go belly up, which they very sadly can. Look at the Relationships board on any given day. It's full of women who never thought they'd be where they are right now.
I absolutely respect people's right never to marry - there are times when it would be a bad idea - which is precisely my problem with 'if you cohabit for X years' alternative. It fucks over people's rights to cohabit and not marry. Nobody should be entered into a legal contract by stealth, it should be something that you choose to do.
The law has no way of knowing whether two people both want their relationship to be legitimised, unless they have legitimised it. People who say "well we've been together for 40 years" are missing the point. It's not about how in love you are, it's simply whether both of you have willingly walked into the office and said, "We want to commit legally to each other and legalise our relationship". If one person doesn't want to do that (or both), there's probably a reason and it's not for the law to make the decision for them.
In the case of child-related benefits such as this particular one, though, there does seem to be a case for reviewing them. Once again, PaulDacreRimsGeese made an excellent post about it.