As a privacy-concerned user, a bookmarklet may be preferable because it's only going to execute code when you explicitly request it, rather than an extension running all the time.
That and you know its code won’t change. Vet it yourself once and then use with peace of mind.
Compare to a browser extension which wants permission to read and modify content on all websites, and by default is set to auto-update (in Firefox at least). Even if the extension author is trustworthy and the code is benign to begin with, how do you know it’ll stay that way? Extensions have been sold before for nefarious purposes.
