Don't spend too much time worrying about this. A joint account will not make or break an application. The VO is looking for indicators of a bona fide, conjugal relationship when they assess each application. A joint account can be considered "towards" the assessment but it is certainly not the only thing they look at. Not by a long shot!
My husband and I don't have a joint account and probably never will! That's just the way we live and manage our finances. It's a personal choice we made for various reasons, all of which make complete sense and the fact that we discussed and decided this proves MORE towards genuineness than a piece of paper with two names on it.
Look, not saying it is wrong to do it if it works for you as a couple. But looking contrived can be dangerous as well. So, the original question was How important is a joint account? On a scale of 1 to 10 I'd give it a 3.