Yodlee's a big screenscraping shop (I remember rumors of having large teams dedicated to just keeping the scripts updated). I don't see how any delegated credentials could work in that case.
Mint has switched to Intuit's backend (there was a thread on Quora about this) but I doubt their approach is any different since a lot of banks just don't offer any OFX/other APIs.
I think the argument is that Mint would not need to store the users' raw bank credentials. Yodlee does need to hang on to the raw credentials, but Mint (when they were using Yodlee) only needed to pass them through to Yodlee in exchange for a token.
My assumption was that once Yodlee proved they weren't going away, they could strike privileged deals with banks that obviated the need for screen-scraping, to the mutual security benefit of everyone.
Mint has switched to Intuit's backend (there was a thread on Quora about this) but I doubt their approach is any different since a lot of banks just don't offer any OFX/other APIs.