Agarwal's most compelling reason for preferring bookmarklets to extensions (when possible) is browser compatability:
Add-ins are browser specific, so tomorrow if you decide to move from Firefox to Opera or Google Chrome, your favorite add-ins will no longer work. On the other hand, a bookmarklet written for Firefox is very likely to work with Safari or IE.
I'd add that bookmarklets are easily syncable between browsers using something like Foxmarks, while add-ons still aren't easy to sync.
We've covered several of the bookmarklets the post highlights, but they're all very nicely rounded up for easy drag-and-drop "installation" to your bookmarks toolbar. For more bookmarklet fun, check out our 10 favorite bookmarklets.
Your Guide to Most Useful Bookmarklets [Digital Inspiration]