I'm currently doing the same thing for my own personal use. My bank has recently redone their mobile app and it's actually very nice, but I still don't have the insight I'd like without breaking out my calculator.
Where I'm from the banks run on ancient software (we're talking COBOL in most cases) and when an ATM breaks, you see it briefly boot through Windows 98.
Web scrapers could be feasible for an e-commerce service (that's mostly what Yodlee is, the service that powers Mint) the hard part is the regulatory issues surrounding banking web scrapers. It's a very very grey area.
In my opinion with this stuff, if there's demand, it's better to ask for forgiveness than permission and third party banking apps could/can provide endless functionality and insight.