I divorced my ex after being legally separated for two years. For me I just wanted to draw a line under that part of my life. No acrimony, no kids, documents signed all done for about £100.
Another situation, slightly different from yours though, I was seeing a guy (let's call him Mr Shitface) who said he'd split from his wife but until their house was sold, they couldn't afford to not live in it together. I believed him because I would have been a hypocrite not to. My ex and I had lived together, albeit in separate rooms, until our house was sold and we could properly go our separate ways. Unfortunately Mr Shitface was leading a double life and when house was finally sold, he and wife went on to buy another place together so he was spinning me a crock of shit.
I guess it depends on which direction you want your relationship to go it. Personally I wouldn't like to embark on any commitments together, house, kids etc. until he was divorced. Too many contractual obligations.
A friend of mine, despite being divorced and remarried, remained on good terms with her ex-hubby. He ad never gotten round to changing his will so despite living with a new partner, my friend was next of kin and executor of his will. There was mucho hassle between her and the new partner, understandably.