Another thing that I don't think has been mentioned is benefits payable from the state if one of you dies. If you are married at date of death the widow(er) is eligible for some (not sure exactly what!) benefits. Maybe widows allowance and possibly something for any children under 18? If you are not married I doubt you'd get anything and even if you could, you'd have to prove you were financially dependant/living as married - not something you want to be sorting on top of everything else when your spouse dies.
So even for those who don't have any savings or assets there is the state benefits to consider.
With regards to pensions there are still some "types" of pensions that have to be paid to the married spouse on death under legislation. Anything else is payable under the scheme's rules, which may only consider a married spouse as an "eligible spouse".
It is entirely possible (and I have seen it happen) that if a man doesn't divorce, but leaves his wife and has a partner for a long time (say 20 years) and then dies, the spouse's pension could be paid to his legal wife who he hasn't been with for 20 years+, not his long term partner. It is final salary schemes that will have old rules about spouses like this, so if your dp has a final salary scheme, it's worth asking about spouse's benefits on death.
I also think in the situation i mention above, the wife could contact the DWP and receive the benefits from the state.
Marriage isn't just about protecting yourself if the other person screws you over, it's also about protecting yourself if/when something happens to one of you.