That capability has gained the extension an avid following of Web surfers who want to customize the sites they visit, removing design glitches and stripping sites of ads. But the extension comes with substantial security risks, and could stir trouble among site owners who object to individual, custom redesigns of their pages.
What this has done has shed light on the concept of individual users messing with Web pages. Previously, you could have user-supplied stylesheets, which would do sort of the same thing, but Greasemonkey has really catapulted the capability forward.
Scoble and Doctorow have been going back and forth about the concept of custom page re-writes — Doctorow in favor, and Scoble against (more against the basic concept of auto-rewrites built into the Google toolbar, although he approved of them if they followed his manifesto on the subject).
(Since I have written the phrase “Chris Pirillo” in this post, I fully expect him to comment here in the next 24 hours. I’d be interested to hear what he thinks about the above scenario.)