If you would want a webpage to scan a local file (which is not managed by the browser itself, like an actual cookie), you can't count the seconds that any decent anti-virus program needs to declare it as a bad site and deny access. Just imagine a webpage scanning your files. That is unwanted. Also, you can safe your password, etc. using actual cookies and you still have to enter them only once (as long as you don't delete your cookies). It's not really a lot of work to enter your username and password once.
It could also lead to a security issue. What if it is a family computer and someone accidentally clicked the save username/password boxes. Everyone using the computer could access their account. I don't think that would be a good thing.
Overall, I'd say this is an invalid request, which could pretty much only hurt the game, instead of enhance it.