I wasn't going to post this because it contains sweeping generalisations but then I thought, "Hey, when has offending some geeks on a computer forum ever bothered me before?"
Let's get all the group hug stuff out of the way first. Yes, horses for courses, YMMV, what works for one may not work for another blah blah blah. I think we're all clear on this assumption.
However, let's get back to those sweeping generalisations I mentioned earlier. The primary reasons I can see for wishing to maintain separate accounts are :
1. Selfishness - "It's MY money, not ours, MINE!"
2. Trust issues - "She might leave me so I'd better have a nest egg."
3. Deceitfulness - "Heh, she'll never know I bought this new processor."
[/generalisations]
We find a joint account easier to manage and it gives us instant visibility of how our finances are looking at any one time. We don't have any desire to hide the things we buy - we're adults not children - so one account is perfect for us.
Look, I don't suppose it really matters how strangers on a computer forum manage their money. If it works for you then that's just great. But don't think that Castiel and a few others, including myself, are alone in thinking a joint account makes more sense. Remember that this is a computer forum and that attracts a type of person who - generalisations back on - will be more likely to be selfish, inward looking and self-focussed.
So yeah, do what you want with your money I guess.