I don't think financial comingling can be understated. To share finances, you need to trust the person. If you open joint account with a random person, the funds can be depleted by that person without any repercussion... They're a joint holder thus have right to empty out account any moment.
Both parties contributing to account and sharing access to funds to each other is good evidence of bonafide marriage.
Your comment is akin to other comments I've seen on VJ about children born in marriage isn't good evidence because anybody can have a child.
I strongly disagree with these sentiments.
Yes, there are people who do silly things - add strangers to their accounts, have kids with random people. But it's not different from silly things like people signing I-864, sharing insurance or adding somebody on their house deed to only realize they were used.