HarryT,
The reason to not use the browser certainly has nothing to do with whether or not you are already jailbroken.
And saying "Oh, it's OK to use the browser, but only if you know ahead of time you can trust the website in question to not hack your Kindle", is very different from your original statement.
It's also rather limiting, since you don't really know which websites are safe, which ones may get hacked, and in which situations you may fall prey to a man-in-the-middle attack (especially on public WiFi).
...
Using an insecure browser which contains a full-time root exploit is just plain stupid. If Internet Explorer had a publicly known vulnerability that could compromise the administrator account with simple javascript, would you nevertheless continue using Internet Explorer on your PC?
Especially given that Firefox and Chrome exist?