The idea was (if you read his "What Is This Thing" entry (http://jonaquino.blogspot.com/2005/06/yubnub-my-e ntry-for-rails-day-24-hour.html) that it would indeed be similar to FF's keyword feature, but be available from anywhere.

(It's right there under "2. Why did you make YubNub?")

So yes, it's slightly harder to use than a browser keyword, but it is more widely available.