It's definitely a communication and trust issue but the fact is a lot of people don't trust their significant other and aren't very good communicators. These types of arrangements are the result.
There's different ways to skin a cat. I'd agree that if you're married, probably the simplest solution is joint accounts. But some people might have particular reasons for keeping things somewhat separate. What really blows my mind, though, is when you hear about couples with separate finances that split absolutely every little cost of living 50/50 right down the middle, and then end up living vastly different lifestyle because one makes a lot more money than the other. I remember I was out with some friends years ago and picked up on a conversation a couple women were having about that. One of their husbands was on a vacation to Mexico or wherever, and had gone without the wife and kids because their family budget didn't have enough money to cover it, she couldn't afford to pay her half if it was just a couple's trip with the kids left behind, and so the husband fucked off on his own because he makes twice as much as her and had money in the bank. That's just fucked. I met the husband at some point later on, and he was one of the biggest douchebags I've met in a long time. So no surprise I guess.
Personally I gave the special lady friend a supplemental card on my Visa account last year to take care of groceries and other stuff as needed (when she moved in, the deal was supposed to be that she'd buy groceries but otherwise wouldn't pay for any utilities, condo fees, rent etc because she couldn't afford it. But soon she realized she'd struggle to do even that while also paying for her school tuition, car payment, insurance etc). She hardly spends money on anything non-essential, so I'm not concerned she'll go out and do anything stupid. I track all my finances with Quicken, so I'm a little concerned about how we'd integrate our finances in the future without fucking all that up. But that's more about my OCD than any true practical consideration. I'm pretty sure she still has no real idea how much money I have/make, since I don't really talk about it in specifics and don't throw it around, so she's worried that I will get upset if she spends anything on herself. Seeing as I spend about 10k a year on heli/cat skiing trips, I'd have to be a massive hypocrite to get upset at her for spending a few hundred bucks on a couple new outfits every now and again, or whatever.
The only thing that pisses me off is that she won't let me pay off her CC bills or car loan, so she's just pissing money away paying interest that I'm having to make up for. It's really short sighted, but she wants to feel like she's still somewhat financial independent I guess, and it's not really worth the fight.