1. I can drop-down the list of available search engines and see what's available, without having to remember any key-words to do the searches.

2. (and this is the killer feature for me) If a particular search engine is selected (say, IMDB), I can right-click on any word on a page, and one of the context menu options is "Search IMDB for <selection>..."

Also, horizontal real-estate is not much of a concern on a wide-screen monitor - how much of the URL would I really want to see..?