Yep, I can see the value as a thinking tool. There are tools like http://www.bayesia.com/ and http://www.hugin.com/ but they are enterprise pricey and I find them very clunky.

I'd love to know how your ideal tool might work. Do you mind if I drop you an email (from your profile info)?

I'm working on a decision making tool to let you quickly describe reasoning with simple bullet point lists. It aims to be as quick as a making a rough note so that with just a few different types of bullet, you can create belief networks and computable decision models.

The tool is intended for a general audience so it currently uses simple weighted evidence. If you can tell me how you would like to work with probabilities then you might get the tool you want :)