It's not about the format, it's about the access. I'd be fine with a CSV of my bank transactions, even the completely broken pseudo-CSV that my bank outputs. What I need is the ability to pull that CSV with an API call automatically, instead of having to log in to the bank (which takes almost a minute and involves retyping numbers from a mobile device), and navigate to the right place to find the CSV download (which takes another minute).
Basically, the bank UX sucks for my needs - and I'm guessing, for most people's needs[0]. Cynic in me says that the reason I can't get an API access is because somebody then would make an interface that doesn't suck, and the banks would lose the best place they can upsell people with financial products.
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[0] - There's a reason why "preview your balance and last transaction" feature shows up in mobile banking apps these days.