Why pick on Chrome? How does this differ from extensions offered by Firefox or Internet Explorer? They all allow essentially similar behavior. Eventually it comes down to whether you trust the extension.

If I'm running AdBlock it needs to be able to modify the HTML of any page and occasionally update its lists using the Internet. But AdBlock Extra Evil Edition might also be paid by someone to not block their ads or to leave beacons in place.

A form filling application could potentially push evil information into a form and submit it before I could do anything. Or it may work the way I expect and only fill in data that I want to be filled. How do you allow the good stuff without also allowing the bad stuff?