We are in the same position. We have been engaged - ahem - for 28 years. Had 2 weddings booked over the years but cancelled both due to a) discovering I was up the duff and realising we needed to buy a house asap rather than leisurely saving up for a mortgage deposit and a big wedding; and b) my DF died suddenly a week after I'd booked the registry office and I just couldn't think about it. I almost daren't book a third as gawd knows what might happen! Lottery win perhaps 
Mortgage, life insurance and wills have always been sorted but we are in the same position re pensions. The terms of both mine and DP's says they will pay out to a 'spouse' and the definition of spouse in the t&cs is clearly a someone you are either married to or in a legal civil partnership with. We have both worked too hard and paid in far too much for it just to go back into the pot so we want to make sure that either is entitled to inherited rights if either of us snuff it. At 45 and 47 we are starting to think about this stuff a bit more seriously.
You also have to consider issues like 'next of kin' consent if either of you were in need of emergency medical treatment or even worse end of life care, organ transplantation etc. DS is both mine and DP's next of kin now he's over 18, previously it was our DMs. Technically we wouldn't have a say about each other's treatment and would have to rely on DS - or the DMs previously - to take our wishes into account. But the reality is that either of our DMs - as could DS now - could totally override our wishes as legal next of kin. Not that they would've done but not all families are close, caring or share the same beliefs.
It's a big step whenever you take it but after 18 years, 2 DC, a mortgage etc I think it makes sense. Yes, if you ever split up it makes separation more expensive but if you are in a long term relationship with dependents I think it offers some financial protection and provides next of kin status which can be the most important of all.
I'm planning on probably doing it 2015 to make it a full on 30 yr engagement. DS will be 27 then and I might even make him dress as a page boy! Seriously though, it will be low key, informal and fun rather than the full on big white expensive jobby we originally had planned back in 1989. Oh and DP and I have never had a joint account, ever. We just work things out between us and we've never had a problem.
Sorry that's a bit long. Red wine fuelled tonight!