Bookmarklet Advantages

Visibility of Accessibility Information
Bookmarklets make hidden accessibility information visible in the graphical rendering to web designers, developers and accessibility testers
Manual Checking
Support manual checking of WCAG 2.0 accessibility requirements, inconjunction with other testing tools and proceedures
Browser Compatibility
Can be used in any modern browser (e.g. Chrome, Firefox, Safari, IE 10+)
Administration
Fewer administrative issues than browser extensions/add-ons (e.g. do not need admin support to install in some managed environments)
Accessibility Categories
Bookmarklets break up accessibility issues into functional groupings
Rule categories in OpenAjax Rulesets of AInspector Sidebar and FAE 2.0