It worked for my Mum. She spent 15 years with my stepdad. They kept seperate homes, seperate finances, some times holidayed together but often apart etc.
Both of them had grown to enjoy their own space after their divorces, so it worked for them. They spent some nights at hers, some as his, and some seperately.
They were unmarried for 13 of those years, until my Mums cancer came back and became terminal. They married for the party, and to acknowledge their relationship, I don't think she wanted my Dad to be remembered as her (only) husband.
There was also a financial side to it, as part of her pension was set up to go to her spouse when she died. If she wasn't married, then noone got it. All other assets went to her kids when she died.
My stepdad basically moved in with her for the last nine months of her life in order to care for her, but still kept his own home and would spend the odd night there.
Despite living separately (or maybe because of it!), she said that the years with my Stepdad were the happiest of her life. It certainly didn't seem like any less of a relationship to me or the rest of our family because of their living arrangements.
Had she lived longer, I think they may have ended up moving in together one day, but it was probably a fair way down the road.