I know a few couples like this and I believe, apart from the obvious love, that it is necessary to keep things tidy, wills and the like. I have a friend who, after 20 odd years of living with his partner was diagnosed with cancer. He got married and luckily is now clear.
A lomg story I wont go into but my 2nd wife married her 2nd husband who was very ill at the time and not expected to recover. The marriage was basically to secure her rights as a wife, widows pension etc, after all she gave up work and looked after him for years. Personnaly I would not like to be in a long term relationship without getting married, call me old fashioned ( well not any more joke there somewhere) Best of luck to them.
Both me and my wife have children from previous marriages, it sounds callous, but one reason why we got wed was that the legal situation regarding wills was covered. Love her to bits really but dont tell her.