We had huge problems getting our dd to stop. It did start to affect us negatively in that she was big and wriggly and waking me up several times a night and it had gone on far too long.
You think it doesn't affect your relationship with your dp, but in all honesty I'd say it has for us. It's just the lack of alone time more than anything. She was with us (usually sat in between us) 24/7.
Dd was hugely resistent to stopping when younger. We spent weeks sometimes putting her back to bed, sitting in the room sometimes, sitting outside on the landing (where more than once I eventually fell asleep and she just walked over me and got into bed with dh).
She was probably 8/9 when I just got nasty with her I'm afraid to say. We'd tried rewards, bribery, repeatedly putting her back. It just didn't work. In the end I got to the end of my tether. I'd had nearly 10 years of never having a full night's sleep and feeling exhausted.
I just went no tolerance and told her I'd be putting a lock on her door and ours if she didn't stop because it was making me ill. I also told her she would be going on no sleepovers and no school residential trip until she stayed in her own bed every night. And that I would take a pound out of her money box for every night she entered our room before 7am. (I was desperate by then).
She is by nature a stubborn dc - one who fought sleep, wouldn't eat, wouldn't potty train (even though she was dry day and night, she refused point blank to use a potty or toilet). She still refuses to hold a knife and fork in the right hands now. You have to wait with her until one of her peers makes fun of her, only then will she decide that she'll do what everyone else is doing. She is currently refusing to learn her times tables, because she "doesn't need to". I'm not that worried because when she feels she needs to (e.g. gets demoted a maths group), she'll learn them overnight. But nothing we do/say will make it happen.
Anyway, she pretty much did it overnight then - which made me think she could have done it 6 years earlier no problem at all. She had no nightmares and was not distressed. She just much prefers to snuggle up to someone warm, a bit like a cat and she doesn't like to be left out of a party. I'm quite sure when she's a student she will be the last one to go to bed because she won't want to miss anything.
My advice is to bite the bullet now and not let it drag on. Mine would still be in our bed at age 10.5 if we hadn't managed to stop her. As they get older they need to be more independent. It's a real shame if they can't go on the school/scouts residential trips. One or two of her friends are in that position now.